docs: remove warnings, part 1

[jchavarri]

Part of #693.

Still a long way to go, there are lots of other warnings in compiler libs, e.g.

File "vendor/melange-compiler-libs/lib/parse.mli", line 87, characters 29-43:
Warning: Failed to resolve reference unresolvedroot(Longident).t Couldn't find "Longident"

And also in melange itself, due to the way Js_internal is included I believe:

Warning, resolved hidden path: Js__.Js_internal.Fn.arity2

There are also other warnings that I have no clue how to solve about alerts, e.g.:

File "jscomp/stdlib/moreLabels.mli", line 63, character 2 to line 65, character 3:
Warning: Alert unsynchronized_access not expected here.

But this PR already includes some fixes, maybe can be considered to merge to keep doing progressive improvements.

[anmonteiro]

• re: Js_internal stuff, I think we can probably add some @canonical annotations, e.g.
  https://github.com/ocaml-multicore/eio/blob/19c42eb7a4b763c6307ccf2400624eb25f9b732a/lib_eio/eio_mutex.mli#L13

• I think we can remove some of the unsynchronized_accessalerts from our stdlib

[jchavarri]

thank you sir 🎩