docs: document three NIR_ envvars

docs: document three NIR_ envvars

Initially I was only interested on documenting NIR_PRINT, as today I
needed to check the code to find this envvar, that at the moment I
vaguely remembered that existed.

As we are here, though, let's just document all of them (assuming that
makes sense).

Reviewed-by: Eric Anholt <eric@anholt.net>