egl: Mention if swrast is being forced

egl: Mention if swrast is being forced

The system can be disabling HW acceleration unbeknown to the user,
leading to a long debug session trying to work out which component is
failing. A quick mention that it is the environment override would be
very useful.

v2: Use more generic "CPU renderer" and so try to avoid jargon.

Reviewed-By: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Acked-by: Martin Peres <martin.peres@linux.intel.com>