Merge from origin/emacs-26

9d6a3ac73a Mention pcase as a fifth conditional form
567cb9046d Overhaul pcase documentation
4d7e54acff Use EXPVAL in docstrings of patterns defined using pcase-d...
7e8227ed68 Introduce EXPVAL for pcase, pcase-defmacro docstrings
e6de5b3d51 Ensure pcase doc shows `QPAT first among extensions

diff --cc doc/lispref/control.texi
Simple merge

diff --cc doc/lispref/elisp.texi
Simple merge

diff --cc lisp/emacs-lisp/cl-macs.el
Simple merge

diff --cc lisp/emacs-lisp/eieio.el
Simple merge

diff --cc lisp/emacs-lisp/radix-tree.el
Simple merge

diff --cc lisp/emacs-lisp/rx.el
Simple merge