Add some documentation detailing an aspect of dialplan functions, as requested by Russell

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@77838 65c4cc65-6c06-0410-ace0-fbb531ad65f3

CHANGES

@@ -33,6 +33,10 @@ Dialplan functions
   * MailboxExists converted to dialplan function
   * A new option to Dial() for telling IP phones not to count the call
     as "missed" when dial times out and cancels.
+  * Added LOCK(), TRYLOCK(), and UNLOCK(), which provide a single level dialplan
+    mutex.  No deadlocks are possible, as LOCK() only allows a single lock to be
+    held for any given channel.  Also, locks are automatically freed when a
+    channel is hung up.
 
 CLI Changes
 -----------

funcs/func_lock.c

@@ -200,8 +200,9 @@ static struct ast_custom_function unlock_function = {
 	.name = "UNLOCK",
 	.synopsis = "Unlocks a named mutex",
 	.desc =
-"Unlocks a previously locked mutex.  Returns 1 if the channel had a lock\n"
-"or 0 otherwise.\n",
+"Unlocks a previously locked mutex.  Note that it is generally unnecessary to\n"
+"unlock in a hangup routine, as any lock held is automatically freed when the\n"
+"channel is destroyed.  Returns 1 if the channel had a lock or 0 otherwise.\n",
 	.syntax = "UNLOCK()",
 	.read = unlock_read,
 };