* symfile.h (struct symfile_segment_data): Doc fixes.

* symfile.h (struct symfile_segment_data): Doc fixes.
* symfile.c (symfile_map_offsets_to_segments): Doc fixes.
Assert that we were passed some loaded segment addresses,
and that sections' segment numbers are valid.
Simplify offset calculation.
* remote.c (get_offsets): Clarify selection of relocate-by-segment
strategy, and set num_segments correctly.  Delete redundant
assignments to do_sections.