thrift/rust: add usage message for Rust options
thrift/rust: add usage message for Rust options

Summary:
Add usage message documentation for mstch_rust options.

#forcetdhashing

Reviewed By: dtolnay

Differential Revision: D28909360

fbshipit-source-id: 94604db7718b22fb68db5c726504078d790a3726

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift