How to change comment text using hooks Under each node there is a 'Add Comment' text. For a particular content type I want to change the text to 'review by adding comment". How do I do this?
Try with the String Overrides module. Here is a quote about it (from the module's project page):

Provides a quick and easy way to replace any text on the site.
Features

Easily replace anything that's passed through t()
Locale support, allowing you to override strings in any language
Ability to import/export *.po files, for easy migration from the Locale module
Note that this is not a replacement to Locale as having thousands of overrides can cause more pain then benefit. Use this only if you need a few easy text changes.