Better POD for newSVrv

Better POD for newSVrv

It takes me a couple times to slowly read the description to realize
what this does. The "its reference count is 1" can be misread to mean
someone must mortalize the return value of newSVrv. The "Creates
a new SV for the RV, C<rv>, to point to." can be misread to mean that
newSVrv is a macro, and will do an &() on rv to create a total of
two new SVs. An SvRV pointing to a posssibly blessed undef SV.
This is also wrong. Adding "existing" makes the point that rv
will not be created in this call.