Merge from origin/emacs-26

Merge from origin/emacs-26

e3f00f5 (origin/emacs-26) Clarify when to use advice-add vs add-function
c0f3e6b Minor formatting tweak in the Emacs manual
fadf653 Update comments in Intro to Emacs Lisp
52ccb24 ; * doc/os.texi (Batch Mode): use = after --eval
d65430f * etc/NEWS: Mention change in `edebug-prin1-to-string'.
ef4aec1 ; * doc/lispref/hash.texi (Defining Hash): Fix typos.
2981952 Another attempt to fix sql.el
b1b96d7 Update binding of 'M-.' in Intro to Emacs Lisp
b239a09 Fix a typo in last change in sql.el
0d8bae5 Fix capitalization of mail headers
845fe03 Fix buffer names in sql.el
a3885f5 Minor fixes in the Emacs manual
542f830 Fix a typo in rmail.texi
eb0bc6f * etc/PROBLEMS: Document Bug#31305.

Conflicts:
	doc/emacs/sending.texi
	etc/NEWS