Add implicit symbols section to lex.dd

Having string in the keywords list didn't sit well with me, so I ended
up breaking it out and adding the others as well.  Also added two more
of the double-underscore keywords (that don't appear to otherwise have
documentation).
Document escape feature for ddoc.
Merge pull request #272 from AndrejMitrovic/DocumentEscapes

Document escape feature for ddoc.
Merge pull request #269 from Wyatts/predef

Add implicit symbols section to lex.dd