help: improve grammar/wording of 'extensions' topic

help: more improvements for the extensions topic - improve help text English (thanks to timeless for corrections) - rename and simplify functions a little bit, improved comments

extensions: use _order to determine if an extension is enabled

extensions: check for path existence only when necessary

extensions: remove now-useless pathdirs() function

mq: fix error message for qpush inexistent-patch (issue1702)

highlight: was broken since 580a79dde2a3 (encoding)

help: add/fix docstrings for a bunch of extensions

extensions: simplify by selecting primary hgext

extensions: move extensions listing functions from mercurial.help Includes some small fix-ups to comments in enabled() and disabled().