FindRuby: Add documentation for deprecated variables

Since commit e672db628b (FindRuby: Rename variables to match case of
module name, 2020-03-11, v3.18.0-rc1~546^2), the upper-case-prefixed
variable names are for compatibility only but still exist.  Put them
back in the documentation.

Issue: #21064

Modules/FindRuby.cmake

@@ -48,10 +48,21 @@ This module will set the following variables in your project:
   Ruby patch version.
 
 
-Also:
+The following variables are also provided for compatibility reasons,
+don't use them in new code:
 
+``RUBY_EXECUTABLE``
+  same as Ruby_EXECUTABLE.
+``RUBY_INCLUDE_DIRS``
+  same as Ruby_INCLUDE_DIRS.
 ``RUBY_INCLUDE_PATH``
-  same as Ruby_INCLUDE_DIRS, only provided for compatibility reasons, don't use it
+  same as Ruby_INCLUDE_DIRS.
+``RUBY_LIBRARY``
+  same as Ruby_LIBRARY.
+``RUBY_VERSION``
+  same as Ruby_VERSION.
+``RUBY_FOUND``
+  same as Ruby_FOUND.
 
 Hints
 ^^^^^