Hi,

The following patch clarifies some grey areas and fixes some errors in 
mdoc.7.

(1) Properly escaping "." (in several areas).
(2) Non-macro lines properly referred as "text lines" (earlier referred 
to as "free-form", "non-macro", "other", etc.).
(3) The language in the "font scopes" \f section improved.
(4) Note that a comma should be added to multiple `Nm's in SYNOPSIS.
(5) Note ordering of SYNOPSIS section stuff.
(6) Add more info on DESCRIPTION syntax.
(7) Bl can also contain a HEAD.
(8) Make some `Cm' -> `Ar' (jmc, I'm not 100% sure whether it's the 
other way round: I use the `Ar' when documenting either the parameters 
to a macro, like `\&Os Op Ar osname' or the parameters to a macro 
argument, e.t., `\&Rv \-std Op Ar utility')

Thoughts?

Kristaps