Review project documentation

This issue works as an index to track and discuss progress on improving the documentation of the Twitter module.

These tasks are tagged with a "Needs documentation" tag and are the following ones:

• #1343680: Review README.txt of 6.x-3.x and 7.x-3.x branches
• #1343694: Add Doxygen blocks to every function
• #1344748: Review and extend the online documentation
• #1344898: Create a list of references in blogs and other pages
• #1344908: Create a list of related modules at the online documentation.