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 97.28%. Comparing base (c4c448d) to head (914633b).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #542   +/-   ##
=======================================
  Coverage   97.28%   97.28%           
=======================================
  Files          43       43           
  Lines        1730     1730           
=======================================
  Hits         1683     1683           
  Misses         47       47           

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Before [c4c448d]	After [58fe965]	Ratio	Benchmark (Parameter)
12.7±0.01s	12.8±0.07s	1.01	benchmarks.time_create_large_catalog
27.2±0.4ms	27.6±0.7ms	1.01	benchmarks.time_polygon_search
1.89±0.02s	1.89±0.03s	1	benchmarks.time_create_midsize_catalog
82.3±2ms	82.6±1ms	1	benchmarks.time_kdtree_crossmatch
14.7±0.6ms	13.9±0.2ms	0.95	benchmarks.time_box_filter_on_partition

Click here to view all benchmarks.