isl_pw_*_gist*: update documentation
isl_pw_*_gist*: update documentation

This was missing from isl-0.13-142-g17279c1d5e (isl_pw_*_gist:
special case context equal to single domain,
Fri May 9 15:56:31 2014 +0200).

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl