add some docs

util.c

@@ -5698,6 +5698,18 @@ S_xs_version_bootcheck(pTHX_ SSize_t items, SSize_t ax, const char *xs_p,
     }
 }
 
+/*
+=for apidoc api_version_check
+
+Used by the PERL_API_VERSION_CHECK macro to compare the perl the
+object was built with and the perl that C<libperl> was built with.
+
+This can be used to ensure that these match and produces a more
+diagnosable than random crashes and mis-behaviour.
+
+=cut
+*/
+
 void
 Perl_api_version_check(size_t interp_size, void *v_my_perl,
                        const char *api_version) {