gallium-docs: Add documentation for threading requirements
gallium-docs: Add documentation for threading requirements

Add documentation for the requirements related to threading
for screens and contexts.

Signed-off-by: Axel Davy <2b6ef604b998fb81662f85c329e4fc020eae8e5d@ens.fr>
Reviewed-by: Marek Olšák <8c7344a1abdb103e79ecfd488098373070c3c70e@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler