doc: Clarification for -Wmissing-field-initializers

The manual is incorrect in saying that the option does not warn
about designated initializers, which it does in C++.  Whether the
divergence in behavior is desirable is another thing, but let's
at least make the manual match the reality.

	PR c/39589
	PR c++/96868

gcc/ChangeLog:

	* doc/invoke.texi: Clarify that -Wmissing-field-initializers doesn't
	warn about designated initializers in C only.