Rework the documentation of Spaces, using the Ossanna/Kernighan/Ritter

Rework the documentation of Spaces, using the Ossanna/Kernighan/Ritter
Heirloom Nroff/Troff User's Manual at the authoritative reference.
Part of our text was outright wrong.

Also, refrain from advertising the paddable non-breaking space `\~'
in the DESCRIPTION, for three reasons:  For nroff mode, -Tascii, and
fixed width fonts in general, it makes no difference, so keep the
discussion simple.  Compared to `\ ', '\~' is of questionable portability.
And if you want to keep words together, it is also more usual that you
don't want padding to intervene either.

Finally, drop the `\c' escape sequence (interrupt text processing)
which is not a special character but an input processing instruction
akin to the \<newline> escape sequence.