Don't apply shorthands to punctuation-only symbols (bug#51089)

Don't apply shorthands to punctuation-only symbols (bug#51089)

This includes symbols used for arithmetic functions such as -, /=,
etc.  Using "-" or "/=" is still possible but doing so won't shadow
those functions.

* doc/lispref/symbols.texi (Shorthand, Exceptions): New
subsubsection.

* src/lread.c (read1): Exempt punctionation-only symbols from
oblookup_considering_shorthand.

* test/lisp/progmodes/elisp-mode-tests.el
(elisp-dont-shadow-punctuation-only-symbols): Tweak test.