Add new format for literal key sequences to substitute-command-keys

Add new format for literal key sequences to substitute-command-keys

* lisp/help.el (substitute-command-keys): Add new format "\\`f'" for
literal key sequences.  (Bug#50804)
* doc/lispref/help.texi (Keys in Documentation): Document the above
new substitution.

* test/lisp/help-tests.el
(help-tests-substitute-command-keys/literal-key-sequence):
(help-tests-substitute-command-keys/literal-key-sequence-errors): New
tests.
(help-tests-substitute-key-bindings/face-help-key-binding): Extend test.