Merge from origin/emacs-26

Merge from origin/emacs-26

12c77f6 (origin/emacs-26) Add ido-fallback special variable (Bug#31707)
826e8d1 Merge branch 'emacs-26' of git.sv.gnu.org:/srv/git/emacs into...
c784876 Tighten a cross-reference in documentation
517dc0b Fix last change in tramp-sh.el
f43186f Revert previous patch; comment was OK after all.
4c3306e Fix lead comment for count_trailing_zero_bits
b419f27 ; * doc/emacs/files.texi (Interlocking): Fix a non-portable @...
7488de4 * lisp/emacs-lisp/regexp-opt.el (regexp-opt): Fix docstring q...
0b69807 Make a minor update to the CSS mode docstring
9a53b6d Say how to override a primitive interactive spec
1d77078 Fix Bug#31941