[Groff] EQN documentation

[Cedric Sodhi]

I'd like to suggest that a documentation for EQN is implemented in
groff. Currently, there is only a very short manualy which describes
differences between Groff and Roff EQN. For a useful you have too google
for the original UNIX manpages which are, by the way, only hosted very
sparely over the internet. And even there the documentation of EQN
appears to be incomplete.
Occasionally I stumble over features such as the "grad" operator, but I
find out about those only by accident. So far, I'vent found any
reference which mentions, even remotely, that something like a shortcut
for the Nabla exists.