Remove two uses of old if-then-else syntax from the REFERENCE

Rework grammar infrastructure, losing 'preference' for concrete syntax.

Instead, the most recent rule is the one the pretty-printer will prefer.
The clear_prefs function disappears, but the prefer_form function stays
because it can just adjust the timestamp on the desired form to be most
recent.

Huge number of check-ins, implementing change to .doc file syntax.
Now the rule for text within braces is that it appears as given, but
that braces must balance. If you want an unbalanced brace, you can use
\lbrace or \rbrace. No existing .doc file uses unbalanced braces.
Also, there is now no need to use \noindent. In fact, doing so
will get you a complaint from the parser. Similarly, use of \ or
{} characters outside of verbatim text blocks will cause griping.
All of the new files have been generated by a parse-then-print process
from the old files, and it's possible there are errors. If you ever
have a spare moment, casting your eye over a random selection of these
might help to find errors.

New versions of docfiles.