Revert "mesa: remove unnecessary 'sort by year' for the GL extensions"

This reverts commit 3d81e11b.

As reported by Federico, some games require the 'sort by year' since
they truncate the extensions which do not fit the fixed size string
array.

Seemingly I did not consider that, as the documentation (both Mesa and
Nvidia) mentions about program crashes ... which are worked around by
setting the env. variable.

This commit reinstates the workaround and enhances the documentation.

Cc: Marek Olšák <maraeo@gmail.com>
Cc: Ian Romanick <idr@freedesktop.org>
Reported-by: Federico Dossena <info@fdossena.com>
Fixes: 3d81e11b ("mesa: remove unnecessary 'sort by year' for the GL
extensions")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Tested-by: Federico Dossena <info@fdossena.com>

(cherry picked from commit fe8be81b)