mesa: simplify MESA_GL_VERSION_OVERRIDE behavior of API override

mesa: simplify MESA_GL_VERSION_OVERRIDE behavior of API override

v2:
 - Provide a correct explanation on the envvars documentation (Ian).
 - Provide a more correct explanation on the function comments (Andres).
v3:
 - Homogenize documentation and inline comments (Emil).
 - Correct a typo (Emil).

Fixes: 2599b92eb97 ("mesa: allow forcing >=3.1 compatibility contexts
with MESA_GL_VERSION_OVERRIDE")

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Ian Romanick <ian.d.romanick@intel.com>
Cc: Eric Engestrom <eric.engestrom@imgtec.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>