Document that we recommend to turn off -gmodules when building a static
library for distribution to other machines on the clang man page.

llvm-svn: 256287
diff --git a/clang/docs/CommandGuide/clang.rst b/clang/docs/CommandGuide/clang.rst
index 9f6d6f5..2b45695 100644
--- a/clang/docs/CommandGuide/clang.rst
+++ b/clang/docs/CommandGuide/clang.rst
@@ -262,7 +262,12 @@
   Generate debug information that contains external references to
   types defined in clang modules or precompiled headers instead of
   emitting redundant debug type information into every object file.
-  This option implies `-fmodule-format=obj`.
+  This option implies :option:`-fmodule-format=obj`.
+
+  This option should not be used when building static libraries for
+  distribution to other machines because the debug info will contain
+  references to the module cache on the machine the object files in
+  the library were built on.
   
 .. option:: -fstandalone-debug -fno-standalone-debug