Is it legal to use content from Apple developer site for internal training documents?
Well, not a lawyer here, but you could infer that your use would be considered acceptable use based on the following information, as long as you don't use any paid content (content obtained as part of a paid developer program) in these presentations:  


Support Center - Registered Apple Developers has a question titled "What are the terms and conditions of being a Registered Apple Developer?", the answer for which points to the Registered Apple Developer agreement (PDF).
The agreement PDF talks about publicly available information and confidentiality here:


  4. Confidentiality.
  You agree that any Apple pre-release software and/or hardware (including related documentation and materials) provided to you as a Registered Apple Developer (“Pre-Release Materials”) and any information disclosed by Apple to you in connection with Apple Events or Paid Content (defined below) will be considered and referred to as “Apple Confidential Information”. Notwithstanding the foregoing, Apple
  Confidential Information will not include: (i) information that is generally and legitimately available to the public through no fault or breach of yours, (ii) information that is generally made available to the public by Apple, (iii) information that is independently developed by you without the use of any Apple Confidential Information, (iv) information that was rightfully obtained from a third party who had the right to transfer or disclose it to you without limitation, or (v) any third party software and/or documentation provided to you by Apple and accompanied by licensing terms that do not impose confidentiality obligations on the use or disclosure of such software and/or documentation. 

Note the exclusion of "(ii) information that is generally made available to the public by Apple" from the list of Apple Confidential Information.
Also, when you browse through the documentation online (or download a PDF), they don't state those as Apple Confidential Information. However, you are allowed to disclose Apple Confidential Information to other employees/contractors who are also registered Apple developers.
The agreement PDF also deals with Apple Confidential Information disclosure here:


  5. Nondisclosure and Nonuse of Apple Confidential Information. Unless otherwise expressly agreed or permitted in writing by Apple, you agree not to disclose, publish, or disseminate any Apple Confidential Information to anyone other than to other Registered Apple Developers who are employees and contractors working for the same entity as you and then only to the extent that Apple does not otherwise prohibit such disclosure. Except for your authorized purposes as a Registered Apple Developer or as otherwise expressly agreed or permitted by Apple in writing, you agree not to use Apple Confidential Information in any way, including, without limitation, for your own or any third party’s benefit without the prior written approval of an authorized representative of Apple in each instance. You further agree to take reasonable precautions to prevent any unauthorized use, disclosure, publication, or dissemination of Apple Confidential Information. You acknowledge that unauthorized disclosure or use of Apple Confidential Information could cause irreparable harm and significant injury to Apple that may be difficult to ascertain. Accordingly, you agree that Apple will have the right to seek immediate injunctive relief to enforce your obligations under this Agreement in addition to any other rights and remedies it may have. If you are required by law, regulation or pursuant to the valid binding order of a court of competent jurisdiction to disclose Apple Confidential Information, you may make such disclosure, but only if you have notified Apple before making such disclosure and have used commercially reasonable efforts to limit the disclosure and to seek confidential, protective treatment of such information. A disclosure pursuant to the previous sentence will not relieve you of your obligations to hold such information as Apple Confidential Information. 

The agreement prohibits you from sharing paid content available as part of a paid developer program with anyone (including other employees in your organization) here:


  7. Paid Content License and Restrictions. As a Registered Apple Developer, you may have access to certain proprietary content (including, without limitation, video presentations and audio recordings) that Apple may make available to you from time to time for a separate fee (“Paid Content”). Paid Content shall be considered Apple Confidential Information, unless otherwise agreed or permitted in writing by Apple. You may not share the Paid Content with anyone, including, without limitation, employees and contractors working for the same entity as you, regardless of whether they are Registered Apple Developers. Subject to these terms and conditions, Apple grants you a personal and nontransferable license to access and use the Paid Content for authorized purposes as a Registered Apple Developer; provided that you may only download one (1) copy of the Paid Content and such download must be completed within the time period specified by Apple for such download. Except as expressly permitted by Apple, you shall not modify, translate, reproduce, distribute, or create derivative works of the Paid Content or any part thereof. You shall not rent, lease, loan, sell, sublicense, assign or otherwise transfer any rights in the Paid Content. Apple and/or Apple’s licensor(s) retain ownership of the Paid Content itself and any copies or portions thereof. The Paid Content is licensed, not sold, to you by Apple for use only under this Agreement, and Apple reserves all rights not expressly granted to you. Your rights under this license to use and access the Paid Content will terminate automatically without notice from Apple if you fail to comply with any of these provisions.