Document Clang's support for #pragma comment(lib/linker) with -fms-extensions

As suggested by Dmitri Gribenko.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181433 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/UsersManual.rst b/docs/UsersManual.rst
index 3dc07ab..e881f15 100644
--- a/docs/UsersManual.rst
+++ b/docs/UsersManual.rst
@@ -1202,6 +1202,12 @@
    record layout. GCC also contains support for this feature, however
    where MSVC and GCC are incompatible clang follows the MSVC
    definition.
+-  clang supports the Microsoft ``#pragma comment(lib, "foo.lib")`` feature for
+   automatically linking against the specified library.  Currently this feature
+   only works with the Visual C++ linker.
+-  clang supports the Microsoft ``#pragma comment(linker, "/flag:foo")`` feature
+   for adding linker flags to COFF object files.  The user is responsible for
+   ensuring that the linker understands the flags.
 -  clang defaults to C++11 for Windows targets.
 
 .. _cxx: