Update the documentation about labeled (locked) narrowing

Update the documentation about labeled (locked) narrowing

* src/xdisp.c (syms_of_xdisp) <fontification-functions>: Update
docstring.

* src/keyboard.c (syms_of_keyboard) <pre-command-hook>:
(syms_of_keyboard) <post-command-hook>: Update docstring.

* src/editfns.c:
(narrowing_locks): Explain why an alist is used instead of a
buffer-local variable.
(reset_outermost_narrowings): Point to recipes that demonstrate
why it is necessary to restore the user narrowing bounds when
redisplay starts.
(Fwiden): Update docstring.
(Fnarrow_to_region): Update docstring.
(Finternal__lock_narrowing): Update docstring.
(Finternal__unlock_narrowing): Update docstring.
(Fsave_restriction): Update docstring.

* src/buffer.c (syms_of_buffer)
<long-line-optimizations-region-size>: Update docstring.
(syms_of_buffer) <long-line-optimizations-bol-search-limit>:
Update docstring.

* lisp/subr.el (with-narrowing): Update docstring.
(without-narrowing): Update docstring.

* etc/NEWS: Mention the 'long-line-optimizations-region-size' and
'long-line-optimizations-bol-search-limit' options.
Announce the 'with-narrowing' and 'without-narrowing' forms.

* doc/lispref/positions.texi (Narrowing): Update the documentation
of 'narrow-to-region', 'widen' and 'save-restriction'.  Document
the 'with-narrowing' and 'without-narrowing' special forms.

* doc/lispref/display.texi (Auto Faces): Update the documentation.

* doc/lispref/commands.texi (Command Overview): Document the fact
that the buffer is narrowed around 'pre-command-hook' and
'post-command-hook' when the buffer text includes very long lines.