PRESENCE_STATE: Provide better documentation for the 'e' option.

Notes that the 'e' option actually decodes data when used as a write function
such as with the SET application while it encodes data when used to read.

Review: https://reviewboard.asterisk.org/r/2335/
........

Merged revisions 381655 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@381656 65c4cc65-6c06-0410-ace0-fbb531ad65f3