Document uvuni_to_utf8()

This is deprecated, but we should give enough information to tell people
what to do.  And this moves it to the public API so that its deprecation
gets more prominence.

Author: khwilliamson

embed.fnc

@@ -3890,7 +3890,7 @@ Admp	|U8 *	|uv_to_utf8_msgs|NN U8 *d				\
 				|UV uv					\
 				|UV flags				\
 				|NULLOK HV **msgs
-CDbp	|U8 *	|uvuni_to_utf8	|NN U8 *d				\
+ADbdp	|U8 *	|uvuni_to_utf8	|NN U8 *d				\
 				|UV uv
 EXdpx	|bool	|validate_proto |NN SV *name				\
 				|NULLOK SV *proto			\

mathoms.c

@@ -142,6 +142,15 @@ Perl_utf8_to_uvuni(pTHX_ const U8 *s, STRLEN *retlen)
     return NATIVE_TO_UNI(valid_utf8_to_uvchr(s, retlen));
 }
 
+/*
+=for apidoc_section $unicode
+=for apidoc uvuni_to_utf8
+
+Instead use L<perlapi/uv_to_utf8>.
+
+=cut
+*/
+
 U8 *
 Perl_uvuni_to_utf8(pTHX_ U8 *d, UV uv)
 {