runtime(doc): mention option-backslash at :h CompilerSet

Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: chrisbra

runtime/doc/usr_51.txt

@@ -1,4 +1,4 @@
-*usr_51.txt*	For Vim version 9.1.  Last change: 2024 Apr 04
+*usr_51.txt*	For Vim version 9.1.  Last change: 2024 Nov 13
 
 		     VIM USER MANUAL - by Bram Moolenaar
 
@@ -656,6 +656,8 @@ an example: >
   CompilerSet errorformat&		" use the default 'errorformat'
   CompilerSet makeprg=nmake
 
+Note: arguments need to be escaped according to |option-backslash|.
+
 When you write a compiler plugin for the Vim distribution or for a system-wide
 runtime directory, use the mechanism mentioned above.  When
 "current_compiler" was already set by a user plugin nothing will be done.