ruby21.y: track upstream refactoring.

Original commit: https://github.com/ruby/ruby/commit/12a0b0c8f48f168994245198e62d1d40cab5d594
Add TODO with relevant upstream commits.
Terminology fixes.
TODO -> TODO.md.
+ lexer.rl (Ruby 2.1): "1end": lex non-exponent `e' separate from number.

Upstream commit: https://github.com/ruby/ruby/commit/41f864faab6f1cdd8ad7e88c9c89fea200d030d2
Documenation: Specify YARD not to display objects marked with @private logic
Documentation: Mark Parser::Source::Buffer::ENCODING_RE as @private