The =apidoc entries for hv_clear() and hv_undef() were a bit spartan.
Make it clearer what the two functions actually do, and the relationship
between them.
/*
=for apidoc hv_clear
-Clears a hash, making it empty.
+Frees the all the elements of a hash, leaving it empty.
+The XS equivalent of %hash = (). See also L</hv_undef>.
=cut
*/
/*
=for apidoc hv_undef
-Undefines the hash.
+Undefines the hash. The XS equivalent of undef(%hash).
+
+As well as freeing all the elements of the hash (like hv_clear()), this
+also frees any auxiliary data and storage associated with the hash.
+See also L</hv_clear>.
=cut
*/