Delete trailing whitespace in help/Docfiles

Thm.ETA_CONV -> Drule.ETA_CONV, Thm.Eta -> Drule.RIGHT_ETA

Cursory profiling indicates that an implementation of ETA_CONV as a primitive
inference rule in Thm is about 20 times faster than an implementation as a
derived rule that instantiates ETA_AX, but that the difference in overall HOL
build time is negligible.

Please do let me know if you encounter (performance) problems due to this
change.

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.

Many fixups, most to do with braces not being doubled properly in a slew
of .doc files. Also, remember to provide an identifier after the
initial \DOC.

Tag and Thm are documented. Plus fixes to existing docs.