[Groff] special characters in groff.7

[Bernd Warken]

The notes in `groff_char.7' are complete now, that makes this
man-page into a useful information center.

The man-page `groff.7' has a section about `special characters'.
This section is not really useful.  It is far from being complete.
I see the following kinds of improvement:

1) Omit all examples there of the type `\(xy' and enlarge the reference to
`groff_char.7', such that `groff_char.7' is enough for all characters
using.

2) Enlarge the examples into something useful, such that `groff_char.7'
is only necessary for special properties.

Bernd Warken