Merge from origin/emacs-26

eb903d8 * lisp/emacs-lisp/pcase.el: Improve docstrings.
86abbb3 * lisp/emacs-lisp/rx.el (rx): Fix typo in doc string.  (Bug#3...
ced58d3 Improve doc string of 'call-process'
38f88a7 Document that generic functions cannot be commands
5aeddfa * lisp/mail/rmailsum.el (rmail-summary-output): Add lost word...
10e0fd8 Add index entries for more isearch commands/bindings (Bug#32990)
de28184 * lisp/simple.el (filter-buffer-substring): Clarify doc (Bug#...
d192c16 Fix recent change in lispref/processes.texi.

diff --cc doc/emacs/search.texi
Simple merge

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

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

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

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

diff --cc lisp/mail/rmailsum.el
Simple merge

diff --cc lisp/simple.el
Simple merge

diff --cc src/callproc.c
Simple merge