[clang][docs] '#pragma clang section' is supported on Mach-O. NFC

This was added back in 7f6e331, but I forgot
to update the docs that referenced it.

Author: jroelofs

clang/docs/LanguageExtensions.rst

@@ -5345,7 +5345,7 @@ The ``#pragma clang section`` directive obeys the following rules:
 
 * The pragma clang section is enabled automatically, without need of any flags.
 
-* This feature is only defined to work sensibly for ELF targets.
+* This feature is only defined to work sensibly for ELF and Mach-O targets.
 
 * If section name is specified through _attribute_((section("myname"))), then
   the attribute name gains precedence.