core: Add some documentation to the malloc_trim code

This adds documentation to handle_cli_malloc_trim() indicating how it
can be useful when debugging OOM conditions.

Change-Id: I1936185e78035bf123cd5e097b793a55eeebdc78

main/cli.c

@@ -1781,7 +1781,19 @@ static char *handle_cli_wait_fullybooted(struct ast_cli_entry *e, int cmd, struc
 
 
 #ifdef HAVE_MALLOC_TRIM
-	/* BUGBUG malloc_trim() is a libc specific function.  Non-portable. */
+
+/*!
+ * \internal
+ * \brief Attempt to reclaim unused heap memory.
+ *
+ * Users have reported that asterisk will sometimes be killed because it can't allocate
+ * more than around 3G of memory on a 32 bit system.
+ *
+ * Using malloc_trim() will help us to determine if it's because there's a leak or because
+ * the heap is so fragmented that there isn't enough contiguous memory available.
+ *
+ * \note malloc_trim() is a GNU extension and is therefore not portable.
+ */
 static char *handle_cli_malloc_trim(struct ast_cli_entry *e, int cmd, struct ast_cli_args *a)
 {
 	extern int malloc_trim(size_t __pad) __THROW;
@@ -1805,6 +1817,7 @@ static char *handle_cli_wait_fullybooted(struct ast_cli_entry *e, int cmd, struc
 
 	return CLI_SUCCESS;
 }
+
 #endif
 
 static char *handle_help(struct ast_cli_entry *e, int cmd, struct ast_cli_args *a);