update gather/scatter docu
update gather/scatter docu

- IndexT is more flexible now
- mention init to 0 for masked gather ctors

VcDevel/Vc,VcDevel/Vc,VcDevel/Vc,chr-engwer/Vc,VcDevel/Vc,chr-engwer/Vc,chr-engwer/Vc