Document and ensure that sv_catpvf() does no argument ordering

sv_catpvf() and friends ultimately end up calling sv_vcatpvfn_flags() with a
C-style va_list argument (rather than with an array of SV pointers). When
the sprintf implementation in sv_vcatpvfn_flags() is called with a va_list
it always ignores any attempt by the format string to reorder the arguments.
This reasonable limitation is now documented, and the implementation throws
an exception when it encounters this situation.

Minimal tests for these exceptions have been added to XS::APItest.

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