[c++]: Improve module-decl diagnostics [PR 98327]

[c++]: Improve module-decl diagnostics [PR 98327]

The diagnostic for a misplaced module decl was essentially 'computer
says no', which isn't the most helpful.  This adjusts it to indicate
what would be acceptable.

	gcc/cp/
	* parser.c (cp_parser_module_declaration): Alter diagnostic
	text to say where is permissable.
	gcc/testsuite/
	* g++.dg/modules/mod-decl-1.C: Adjust.
	* g++.dg/modules/p0713-2.C: Adjust.
	* g++.dg/modules/p0713-3.C: Adjust.