Always add doxygen comments to new functions, more lines than one are

Always add doxygen comments to new functions, more lines than one are
appreciated really. (Read the coding guidelines).

I've worked hard to make chan_sip a better place to code in, let's
keep it that way and don't add more stuff without comments.

Thank you.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@46183 65c4cc65-6c06-0410-ace0-fbb531ad65f3