doc: GL_ARB_buffer_storage is supported on llvmpipe/swr
doc: GL_ARB_buffer_storage is supported on llvmpipe/swr

At least, the extension is exported (gallium capability
PIPE_CAP_BUFFER_MAP_PERSISTENT_COHERENT is 1)

Signed-off-by: Gregory Hainaut <874ed946d2e42378b22d113f3a5c5d90141d0a46@gmail.com>
Acked-by: Edward O'Callaghan <b035da0d7e151e09fe2005f3a7bc526beb9b3245@folklore1984.net>
Reviewed-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler