DOC: Fix improper single backticks in whatsnew
Noticed a few places in whatsnew where items are only surrounded by single backticks, which appears to cause them to render as italic.  For the most part I replaced these with double backticks, e.g. <code>\`Series\`</code> -> <code>\`\`Series\`\`</code>, or removed the backticks if they didn't seem appropriate.
