Rollup merge of #116888 - tbu-:pr_unsafe_env, r=Amanieu

Add discussion that concurrent access to the environment is unsafe

The bug report #27970 has existed for 8 years, the actual bug dates back to Rust pre-1.0. I documented it since it's in the interest of the user to be aware of it. The note can be removed once #27970 is fixed.

Author: matthiaskrgr