Document waterway and potentially other recent tag filtering changes

[koebi]

Since the initial docs-folder was created, not much documentation regarding tag filtering has happened.
At least the waterway:lock_gate-change from #1180 is not documented, #1172 needs documentation as well.

I assume there are some more changes that aren't documented :(

[koebi]

Since the recent overhaul of the documentation, this seems to have been taken into account.