[Groff] .mso for groff_hdtbl(7)

[Bernd Warken]

In order to use hdtbl in a roff source file, the following line
must be used first:

.mso hdtbl.tmac

So it makes sense to include that in the man-page groff_hdtbl(7),
for example at the beginning of the USAGE section.

Moreover its easy to run the examples with groffer from the
command line:

$ { echo '.mso 'hdtbl.tmac; cat; } | sed -e 's/^[ \t]*//' | groffer -

This command waits for input, which you can provide by
marking and copying an example with the mouse.

I like hdtbl, but some more examples would be nice.

Bernd Warken