spelling, grammar, and other simple fixes

The first attached patch contains a bunch of pretty obvious fixes.

The second patch attempts to fix the hierarchy breakage introduced in p6
in -18 while adding the subsections to 3.6. (As can be seen in the table
of contents here:
http://tools.ietf.org/rfcdiff?url2=draft-ietf-httpbis-p6-cache-18.txt).

I think the fix is correct, but I can't get the html/txt versions to
build so I'm not totally sure.

-- Dan