rust_prost_toolchain only accepts a single prost/tonic plugin

The description of prost_plugin and tonic_plugin suggests that they accept more than one, similar to how buf works, but they only accept one.

prost_plugin | Additional plugins to add to Prost.
tonic_plugin | Additional plugins to add to Tonic.

https://bazelbuild.github.io/rules_rust/rust_prost.html

More than one plugin can be combined programmatically, so perhaps the descriptions should be edited to be singular?