Misc. documentation improvements

[AdeelH]

Overview

This PR

• improves function/method docstrings
• renames the BoxList class in rastervision.core.data to NpBoxList to disambiguate it from the BoxList class in rastervision.pytorch_learner (which consequently fixes some incorrect cross-refs in the documentation).

Checklist

• Added needs-backport label if PR is bug fix that applies to previous minor release
• Ran scripts/format_code and committed any changes
• Documentation updated if needed
• PR has a name that won't get you publicly shamed for vagueness

Notes

N/A

Testing Instructions

• Examine doc build.

Completes one of the items in #1505

[codecov]

Codecov Report

Merging #1840 (6376d0c) into master (bff97f3) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1840   +/-   ##
=======================================
  Coverage   80.04%   80.04%           
=======================================
  Files         188      188           
  Lines        9243     9243           
=======================================
  Hits         7399     7399           
  Misses       1844     1844           

Impacted Files	Coverage Δ
...rvision/core/data/raster_source/rasterio_source.py	79.85% <ø> (ø)
.../core/data/raster_source/rasterio_source_config.py	50.00% <ø> (ø)
...tervision/core/data/raster_source/xarray_source.py	92.23% <ø> (ø)
...core/data/vector_transformer/vector_transformer.py	100.00% <ø> (ø)
...tervision/core/evaluation/class_evaluation_item.py	100.00% <ø> (ø)
...astervision_core/rastervision/core/raster_stats.py	99.10% <ø> (ø)
rastervision_core/rastervision/core/utils/stac.py	70.58% <ø> (ø)
...n/pytorch_learner/dataset/visualizer/visualizer.py	96.55% <ø> (ø)
...ch_learner/rastervision/pytorch_learner/learner.py	75.66% <ø> (ø)
...ervision/pytorch_learner/object_detection_utils.py	72.82% <ø> (ø)
... and 8 more