init/Kconfig: add clarification for out-of-tree modules

It doesn't trim just symbols that are totally unused in-tree - it trims
the symbols unused by any in-tree modules actually built.  If you've
done a 'make localmodconfig' and only build a hundred or so modules,
it's pretty likely that your out-of-tree module will come up lacking
something...

Hopefully this will save the next guy from a Homer Simpson "D'oh!"
moment.

Link: http://lkml.kernel.org/r/10177.1469787292@turing-police.cc.vt.edu
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

init/Kconfig

@@ -2080,7 +2080,7 @@ config TRIM_UNUSED_KSYMS
 	  (especially when using LTO) for optimizing the code and reducing
 	  binary size.  This might have some security advantages as well.
 
-	  If unsure say N.
+	  If unsure, or if you need to build out-of-tree modules, say N.
 
 endif # MODULES