lsp.ml comments: move to lsp.mli and follow OCaml doc comment format

Summary:
noop.

Changes:

- Move doc comments about the API of lsp.ml to lsp.mli. The .mli is for documenting the API. Having the doc comments in the .mli is also more consistent with the style used elsewhere in our codebase and in Base (our style guide says to fall back to Base style) and is easier on Merlin/ocaml-lsp iuc.
- Update comments to [OCaml doc comment format](https://v2.ocaml.org/manual/doccomments.html) where the comment seems intended to be a doc comment. This mainly includes beginning doc comments with two asterisks and putting label comments *after* the constructor/field rather than before.
- (sparingly) remove a few of the most redundant comments that just repeat the field name and type. In my experience they make the mind glaze over. I removed that `documentUri` "the uri of the document" but kept borderline-redundant information such as that `DidOpenDocumentParams.textDocument` is "the document that was opened".

Reviewed By: ljw1004

Differential Revision: D45852827

fbshipit-source-id: f79f4888ee28b3bbb817e5a26d612bd8f1688f9d