Avoid dubious uses of save-excursions.

Avoid dubious uses of save-excursions.
* doc/lispref/positions.texi (Excursions): Recommend the use of
save-current-buffer if applicable.
* doc/lispref/text.texi (Clickable Text): Fix the example code which used
save-excursion in a naive way which sometimes preserves point and
sometimes not.
* doc/lispref/variables.texi (Creating Buffer-Local):
* doc/lispref/os.texi (Session Management):
* doc/lispref/display.texi (GIF Images):
* doc/lispref/control.texi (Cleanups): Use (save|with)-current-buffer.
* doc/misc/gnus.texi (Posting Styles): Use with-current-buffer.
* doc/misc/calc.texi (Defining Simple Commands): Prefer save-current-buffer.