Issue/540/docs updates

[OliviaLynn]

Added examples to Catalog's assign docstring, to help users who are less familiar with Dask's assign
Minor spelling/formatting/grammar updates

for #540

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.34%. Comparing base (abfeb5d) to head (48ff75c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #542   +/-   ##
=======================================
  Coverage   97.34%   97.34%           
=======================================
  Files          43       43           
  Lines        1732     1732           
=======================================
  Hits         1686     1686           
  Misses         46       46           

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[github-actions]

Before [abfeb5d]	After [a235b0b]	Ratio	Benchmark (Parameter)
85.7±2ms	88.5±1ms	1.03	benchmarks.time_kdtree_crossmatch
29.9±1ms	30.7±1ms	1.03	benchmarks.time_polygon_search
12.7±0.06s	12.7±0.08s	1	benchmarks.time_create_large_catalog
1.91±0.02s	1.90±0.05s	0.99	benchmarks.time_create_midsize_catalog
15.3±0.5ms	15.0±0.5ms	0.98	benchmarks.time_box_filter_on_partition

Click here to view all benchmarks.

[delucchi-cmu]

Suggested change

	Add a column from a 1-D Dask array:
	Add a column from a 1-D Dask array::

Needed here as well.