Clarify fast-path vs builtin hooks #2405

After reading https://prek.j178.dev/builtin/, I am confused by some of the differences between the lists of fast-path vs builtin hooks:

• If all fast-path hooks are bundled into the prek binary, aren't they by definition builtin, so should be made available in the builtin repo?
• The description of the internal hooks in the builtin repo sound a lot like the fast-path hooks. Shouldn't all builtin hooks that come from pre-commit be fast-path'd?

Fast-path hooks that aren't builtin:

forbid-new-submodules

Builtin hooks that aren't fast-path but are in pre-commit's repo, and aren't noted with a caveat:

check-illegal-windows-names