Heartbleed heuristic gives warning, not failure

Earlier, I had considered making `make test` fail if the version of OpenSSL
being used seemed to be vulnerable to the Heartbleed bug. I was convinced
that this is overly-draconian especially since the library might have been
patched without affecting the reported version number. Therefore, `perl
Makefile.PL` shows a warning if OpenSSL might be vulnerable.

Fix documentation to accurately reflect this.