DateTime: Documentation enhancements.
DateTime: Documentation enhancements.

Update issue 415
Enhanced documentation quite a bit. Basically rewrote all sections in
more detailed level, added more examples, and verified existing.
Documentation is now ready for detailed review.

Also added TODO about deciding what to do with Should Be Date/Time
keywords. They are named inconsistently compared to Should keywords in
other libraries and may actually not be needed in general.

Tasks to do:
- Documentation review.
- Decide what to do with Should Be Date/Time.
- Consider storing dates/times internally using microsecond and not
  millisecond precision.

waldenner/robotframework,waldenner/robotframework,fiuba08/robotframework,ldtri0209/robotframework,ldtri0209/robotframework,fiuba08/robotframework,waldenner/robotframework,ldtri0209/robotframework,ldtri0209/robotframework,fiuba08/robotframework,waldenner/robotframework,fiuba08/robotframework,waldenner/robotframework,fiuba08/robotframework,ldtri0209/robotframework