docs: Improve the rustaceanvim documentation

[cormacrelf]

1. "out of the box" misled me when I skipped to the rustaceanvim section, thought it meant "you don't have to do anything". You do have to add a before_init.
2. Some settings need to be provided in the initparams for rust-analyzer. They are used by people who use non-cargo build systems (hello!). I have updated one of the examples but I think it should maybe be in the main example and all other ones? Other LSPs might need this too.
3. the vim.lsp.config("rust_analyzer"...) was wrong, it's rust-analyzer.

[mrjones2014]

Thanks for contributing!

[cormacrelf]

I've decided I don't think we can assume all LSP servers will tolerate initializationOptions being set to the same value as the configuration request. In the case of RA it is supported and required for some config options to work, but LSP protocol makes absolutely no comment on the contents of initializationOptions so that's not generalizable.

Will clean up tomorrow

[cormacrelf]

+ just applied, and reworded the recommended before_init introduction.