- Fix a few spelling mistakes.

- Add some more documentation for the ast_dynamic_str_............() function
  to document the behavior of the function in the case of a partial write.
  Also, document the return value and note that the function should never need
  to be called directly.


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@48110 65c4cc65-6c06-0410-ace0-fbb531ad65f3