search: tasteful updates to 'language:' filter

Created by: rvantonder

Stacked on https://github.com/sourcegraph/sourcegraph/pull/34947.

Re: RFC 675 decision log, language already exists. But it wasn't documented. We also don't recognize -language in the web app, but it is valid. This PR updates the documentation to prefer language, and fixes the web app diagnostic bug.

Test plan

Cosmetic update and semantics-preserving wrt functionality, so no test.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.