[DOC] RegExp: The Graph property includes some control characters

The behavior of this is a carry-over from Oniguruma:
  https://github.com/kkos/oniguruma/blob/5eaee9f5f8f674aff4875c2b35db00758fa349d6/doc/RE#L246

The previous phrasing was inaccurate since it's fair to construe e.g.
Zero Width Joiner as a control character.

Reported-by: ruby#12294

Author: XrXr

doc/_regexp.rdoc

@@ -836,8 +836,9 @@ Some commonly-used properties correspond to POSIX bracket expressions:
 These are also commonly used:
 
 - <tt>/\p{Emoji}/</tt>: Unicode emoji.
-- <tt>/\p{Graph}/</tt>: Non-blank character
-  (excludes spaces, control characters, and similar).
+- <tt>/\p{Graph}/</tt>: Characters excluding <tt>/\p{Cntrl}/</tt> and <tt>/\p{Space}/</tt>.
+  Note that invisible characters under the Unicode
+  {"Format"}[https://www.compart.com/en/unicode/category/Cf] category are included.
 - <tt>/\p{Word}/</tt>: A member in one of these Unicode character
   categories (see below) or having one of these Unicode properties: