Remove outdated documentation bits concerning -Wmissing-methods

In commit 503219e3, we
stopped suppressing `-Wmissing-methods` warnings on class methods
whose names begin with an underscore. However, it seems the users'
guide documentation concerning this was never updated. Let's do so.

Test Plan: Read it

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #12959

Differential Revision: https://phabricator.haskell.org/D4476

docs/users_guide/glasgow_exts.rst

@@ -11139,7 +11139,7 @@ configurable by a few flags.
                 (and originally defined in ‘GHC.List’))
     where the substitutions are ordered by the order they were defined and
-    imported in, with all local bindings before global bindings. 
+    imported in, with all local bindings before global bindings.
 .. ghc-flag:: -fmax-valid-substitutions=⟨n⟩
     :shortdesc: *default: 6.* Set the maximum number of valid substitutions for
@@ -11215,7 +11215,7 @@ it will additionally offer up a list of refinement substitutions, in this case:
 Which shows that the hole could be replaced with e.g. ``foldl1 _``. While not
 fixing the hole, this can help users understand what options they have.
- 
+
 .. ghc-flag:: -frefinement-level-substitutions=⟨n⟩
     :shortdesc: *default: off.* Sets the level of refinement of the
          refinement substitutions, where level ``n`` means that substitutions
@@ -13778,9 +13778,8 @@ Conjunction binds stronger than disjunction.
 If no ``MINIMAL`` pragma is given in the class declaration, it is just as if
 a pragma ``{-# MINIMAL op1, op2, ..., opn #-}`` was given, where the
-``opi`` are the methods (a) that lack a default method in the class
-declaration, and (b) whose name that does not start with an underscore
-(c.f. :ghc-flag:`-Wmissing-methods`, :ref:`options-sanity`).
+``opi`` are the methods that lack a default method in the class
+declaration (c.f. :ghc-flag:`-Wmissing-methods`, :ref:`options-sanity`).
 This warning can be turned off with the flag
 :ghc-flag:`-Wno-missing-methods <-Wmissing-methods>`.

docs/users_guide/using-warnings.rst

@@ -925,18 +925,6 @@ of ``-W(no-)*``.
     declaration is missing one or more methods, and the corresponding
     class declaration has no default declaration for them.
 
-    The warning is suppressed if the method name begins with an
-    underscore. Here's an example where this is useful: ::
-
-        class C a where
-            _simpleFn :: a -> String
-            complexFn :: a -> a -> String
-            complexFn x y = ... _simpleFn ...
-
-    The idea is that: (a) users of the class will only call
-    ``complexFn``; never ``_simpleFn``; and (b) instance declarations
-    can define either ``complexFn`` or ``_simpleFn``.
-
     The ``MINIMAL`` pragma can be used to change which combination of
     methods will be required for instances of a particular class. See
     :ref:`minimal-pragma`.