Generalize documentation of Face_filtered_graph

[MaelRL]

Summary of Changes

The current doc says that the selection patch IDs need to be of type faces_size_type, but this requirement doesn't exist in the code.

The doc is generalized, and the actual requirement (the type must be Hashable) is added.

TODO:

• Add a few words in the description of the FFG class

Release Management

• Affected package(s): BGL, Stream_support
• Issue(s) solved (if any): fix BGL - add Face_filtered_graph constructor from boolean property map #7570
• Feature/Small Feature (if any): n/a
• License and copyright ownership: no change

[MaelRL]

@janetournois What was the use case where you needed a FFG with Booleans? We could add an example as there are currently 3 examples with FFG, but they are all using a std::size_t value type.

[sloriot]

Successfully tested in CGAL-6.0-Ic-24.

[janetournois]

@janetournois What was the use case where you needed a FFG with Booleans? We could add an example as there are currently 3 examples with FFG, but they are all using a std::size_t value type.

It was for isotropic remeshing of a selection of faces. @ipadjen do you remember where exactly you needed that?

[ipadjen]

It was one of ideas to use it for curvature calculation of a mesh selection. Currently we have the face descriptor for ffg: https://github.com/janetournois/cgal/blob/2bb1e97847204d2ce5acc1025487201f81d7a4dc/Polygon_mesh_processing/include/CGAL/Polygon_mesh_processing/internal/Isotropic_remeshing/Adaptive_sizing_field.h#L65-L72