Add documentation for record_file_append

When this option was added, it was noted in CHANGES, but was missing
the XML documentation that this patch adds.

(closes issue ASTERISK-21780)
Patch-by: Brad Latus (snuffy)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@389202 65c4cc65-6c06-0410-ace0-fbb531ad65f3