doc: clarify meaning of isl_dim_cst
doc: clarify meaning of isl_dim_cst

Since isl_dim_cst is only used to specify the conversion to/from
matrices of constraints, only document it locally.

Requested-by: Uday Kumar Reddy B <5376e33aab6ead4b1b1762a95a6ece9decc8a4e5@iisc.ac.in>
Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

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