doc: prefer @env to @code

Reported by Benno Schulenberg in: http://bugs.gnu.org/18184
* doc/grep.texi: Avoid @code in favor of @env, or of nothing at all.
doc: document LANGUAGE

Reported by Benno Schulenberg in: http://bugs.gnu.org/18185
* doc/grep.texi (Environment Variables): Document LANGUAGE.