pcase docs

[Eli Zaretskii]

Would people who expressed  the desire and intent of contributing
better documentation for pcase please consider doing that in the next
few days?  The initial documentation effort for v21.5 is almost
complete, and we will have our first pretest shortly, so it would be
good to have this item covered when the pretest comes out.

If you are not comfortable with Texinfo, don't bother about the
markup: adding that is a simple job, and I can easily do it.  It's
coming up with good content that takes most of the documentation
effort.

While at that, please also recommend which part(s) of this NEWS entry
needs to be reflected in the ELisp manual:

  ** pcase
  *** New UPatterns `quote', `app', `cl-struct', `eieio', `seq', and `map'.
  *** New UPatterns can be defined with `pcase-defmacro'.

Thanks.