Fix broken RDoc examples that raise or fail to run

The Hash#[] and Array#product examples referenced undefined variables,
the StringIO doc named a nonexistent File::RDRW constant, and
Pathname#mkpath's call-seq advertised a positional permissions
argument that the keyword-only implementation does not accept.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hsbt

doc/stringio/stringio.md

@@ -173,7 +173,7 @@ strio.gets  # Raises IOError: not opened for reading
 Mode specified as one of:
 
 - String: `'r+'`.
-- Constant: `File::RDRW`.
+- Constant: `File::RDWR`.
 
 Initial state: