Misc docs changes

Made a pass through PPI.pm documentation, adjusting references to old revision control systems and development practices. Removed or replaced all references to Parse::Perl as an live project. Contributions/corrections welcome. (All of README.md shows up as modified due to file regeneration changing \r\n to \n.)

Documented the fact that calling PPI::Token::Quote::Double::simplify does not require the caller to also call the document's flush_locations.

Fixed a handful of typos.