[C API] Remove CHECK_OFFSETS and make part of minimal integrity checks

[jeromekelleher]

Currently option 0 means check the integrity of object references, and to check offsets we need to specify CHECK_OFFSETS separately to check whether the offsets in ragged columns are valid. I don't see a good reason for this, and the basic default checks should make sure that the tables at least contain valid memory.

Should do this as part of the process of documenting this API.

See also #596, #592