Fix C-M-a in a C function finding the start of a macro preceding it.

Fix C-M-a in a C function finding the start of a macro preceding it.

Also amend some pertinent documentation.  This fixes bug #23818.

* lisp/progmodes/cc-engine.el (c-beginning-of-decl-1): Also check for a
virtual semicolon at a place where we check for other types of statement ends.

* lisp/progmodes/cc-vars.el (c-macro-nacmes-with-semicolon): Remove from the
doc string the bit saying that the variable is a prototype and liable to
change.

* doc/misc/cc-mode.texi (Macros with ;): Enhance, stating that configuring
macros with semicolon can prevent C-M-a missing the beginning of defun.