docs: update envvar docs to reflect MESA_NO_ERROR change

I changed the behaviour earlier today, but forgot to update the
corresponding docs.

Fixes: 77713a0acb09f475d29f "mesa: allow user to set MESA_NO_ERROR=0"
Suggested-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
1 file changed