docstrings for "point to the x"

## Type of Issue

Select the type of issue:
- [ ] Bug report (to report a bug)
- [ ] Feature request (to request an additional feature)
- [ ] Tracker (I am just using this as a tracker)
- [x] Documentation Asks

## Details

1. Add docstrings to describe how [interpret_point_target](https://github.com/facebookresearch/droidlet/blob/77e9ad05dd68249a7c0fdba13c8b4f853ff5ec5f/base_agent/dialogue_objects/location_helpers.py#L100) works. Specifically,
* why is a 6-tuple returned?
* what is the +1 added to loc[1]?

2. Add docstrings to [compute_location_heuristic](https://github.com/facebookresearch/droidlet/blob/77e9ad05dd68249a7c0fdba13c8b4f853ff5ec5f/locobot/agent/dialogue_objects/spatial_reasoning.py#L36)
* What does it do?
* `rel_dir` is None for pointing (since the action dictionary output by the nsp does not have a `relative_direction` field. How does this work for pointing?