Stasis: Add a usage note on stasis_app_get_bridge

This function returns an ast_bridge without a refcount bump and the
caller must increment the count if it intends to hold the pointer.

(closes issue ASTERISK-23588)
Review: https://reviewboard.asterisk.org/r/3450/
Reported by: Matt Jordan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@412439 f38db490-d61c-443f-a65b-d21fe96a405b