Fix documentation for Emacs 30

Fix documentation for Emacs 30

* etc/NEWS: Mark entries and improve wording.

* doc/emacs/dired.texi (Operating on Files): Document
'dired-do-open'.  Fix indexing.
* doc/emacs/files.texi (Diff Mode): Document 'diff-apply-buffer'.
Fix indexing.
* doc/emacs/maintaining.texi (Project File Commands)
(Project Buffer Commands): Document 'project-any-command'.
* doc/emacs/building.texi (Other GDB Buffers): Document the user
option 'gdb-display-io-buffer'.
(Grep Searching): Document 'grep-use-headings'.
* doc/emacs/text.texi (Outline Visibility): Document Outline
commands that hide/show by heading regexp.
* doc/emacs/killing.texi (Kill Options): Document
'kill-ring-deindent-mode'.
* doc/emacs/basic.texi (Continuation Lines): Improve wording.