Add back documentation for deprecated -Whi-shadowing

This was removed in b538476b, but
without it the compare-flags.py script fails. This adds it back and
marks it as deprecated, with a notice that it is slated for removal.

docs/users_guide/using-warnings.rst

@@ -743,6 +743,23 @@ of ``-W(no-)*``.
 
     This option is on by default.
 
+.. ghc-flag:: -Whi-shadowing
+    :shortdesc: *(deprecated)*
+        warn when a ``.hi`` file in the current directory shadows a library
+    :type: dynamic
+    :reverse: -Wno-hi-shadowing
+    :category:
+
+    .. index::
+       single: shadowing; interface files
+
+    Causes the compiler to emit a warning when a module or interface
+    file in the current directory is shadowing one with the same module
+    name in a library or other directory.
+
+    This flag was not implemented correctly and is now deprecated.
+    It will be removed in a later version of GHC.
+
 .. ghc-flag:: -Widentities
     :shortdesc: warn about uses of Prelude numeric conversions that are probably
         the identity (and hence could be omitted)