doc: add LSP annotations for `lsp_format` options

[mehalter]

This utilizes the Lua language server annotations to document each option and improve the code completion.

Documentation of the option overall:

Completing an option:

[mehalter]

~Also added a commit to fix the type annotation that a function for format_on_save and format_after_save can return nil to not format. I guess technically it could also be "false", but seems more like a lua interpreter side effect and not really the intended use.~

Committed separately upstream, removed this from the PR

[stevearc]

I like this change, but I'll need to add support for nvim_doc_tools to parse and display the alias type before I can merge this.

[mehalter]

Ok! No problem! Do you want me to open a different PR with the other annotation fix in the meantime? Or you could commit it separately or it can wait until your doc tooling support.

[stevearc]

Yeah, I'll make a quick commit to fix the annotations, and will look into the type alias parsing when I get a chance

[stevearc]

docs support added. Thanks for the PR!