Docs: some name links and refs are borked

[goodboy]

Was just re-reading the latest docs and noticed some broken names, namely:

• add_hookspec() (missing a trailing s)
• the call_historic() method ref broken due to module path
• a bunch of private type references that aren't exposed (poke @RonnyPfannschmidt since I know you don't want a lot of these public per say) / linked properly even though we need to document their usage.

It would probably be good if a third party who didn't write the docs went over them and found more issues.

[nightcityblade]

Hi, I'd like to work on this. I'll submit a PR shortly.