Document options for rust-analyzer.completion.callable.snippets

[willhansen]

Looks like this now

Other options aren't documented. Had to search the codebase to find them.

https://github.com/rust-lang/rust-analyzer/blob/4778255b3de9786c59928346470b2ed3cfcdb2c9/editors/code/package.json#L787C16-L787C16