[docs] Add missed specialisations warnings to list of those not enabled by -Wall

Enabling `-Weverything` does enable those warnings.

(cherry picked from commit b062bd10a88ea407ae91610f822f0c352909bcce)
(cherry picked from commit 24b76d1b)

docs/users_guide/using-warnings.rst

@@ -82,6 +82,8 @@ The following flags are simple ways to select standard "packages" of warnings:
         * :ghc-flag:`-Widentities`
         * :ghc-flag:`-Wredundant-constraints`
         * :ghc-flag:`-Wpartial-fields`
+        * :ghc-flag:`-Wmissed-specialisations`
+        * :ghc-flag:`-Wall-missed-specialisations`
 
 .. ghc-flag:: -Weverything
     :shortdesc: enable all warnings supported by GHC