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/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@381655 f38db490-d61c-443f-a65b-d21fe96a405b