Better ad-hoc Emacs release of symbol introduction override
The file etc/symbol-releases.eld now contains explicit version
information for selected symbols that our NEWS* scraper doesn't
resolve correctly.
* etc/NEWS.unknown: Remove this file, replaced with...
* etc/symbol-releases.eld: ...this new file.
* lisp/help-fns.el (help-fns--first-release-override)
(help-fns--mention-first-function-release)
(help-fns--mention-first-variable-release): New.
(help-fns--mention-first-release): Try the override information first
before scraping the NEWS* files.
(cherry picked from commit
59e0b82776ade72680e7c369f6089eab4a74dc4a)