Resolve diagnostic from F::PV::type_hash_from_string when Type: is buggy

It's a class method, not an object/instance method, so it needs to be
passed $filename. Also modernize docs to remind of this fact.
Sync with master, since we might as well put this out in the next release.
Modernize INSTALL and INSTALL.html.
Punctuation improvement
Document why some things are class methods not object/instance methods
OK for release.
Small text fix
OK.  Why didn't anybody comment that this didn't get rolled back to the highest version that had been in the distribution?
fix < and >
Fix spurious underscores