[DOC] Tweaks for String#valid_encoding?

Author: BurdetteLamar

doc/string/valid_encoding_p.rdoc

@@ -0,0 +1,8 @@
+Returns whether +self+ is encoded correctly:
+
+  s = 'Straße'
+  s.valid_encoding?                                 # => true
+  s.encoding                                        # => #<Encoding:UTF-8>
+  s.force_encoding(Encoding::ASCII).valid_encoding? # => false
+
+Related: see {Querying}[rdoc-ref:String@Querying].

string.c

@@ -11469,11 +11469,8 @@ rb_str_b(VALUE str)
  *  call-seq:
  *    valid_encoding? -> true or false
  *
- *  Returns +true+ if +self+ is encoded correctly, +false+ otherwise:
+ *  :include: doc/string/valid_encoding_p.rdoc
  *
- *    "\xc2\xa1".force_encoding(Encoding::UTF_8).valid_encoding? # => true
- *    "\xc2".force_encoding(Encoding::UTF_8).valid_encoding?     # => false
- *    "\x80".force_encoding(Encoding::UTF_8).valid_encoding?     # => false
  */
 
 static VALUE