Update references and documentation of Updater classes

[ekhunter123]

I have updated the documentation for the ChernoffUpdater class. This included editing the documentation to be clearer, and adding another reference. I also specified the parameters and returns of the class methods, following the style of the other update classes.

Second, I added another reference to the PHDUpdater class. This reference specifies the track management system that is used in the class, namely using track tags. This distinguishes it from the tree-based track management algorithms.

[codecov[bot]]

Codecov Report

Merging #683 (1593703) into main (0e8a394) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #683   +/-   ##
=======================================
  Coverage   94.60%   94.60%           
=======================================
  Files         170      170           
  Lines        8544     8544           
  Branches     1652     1652           
=======================================
  Hits         8083     8083           
  Misses        342      342           
  Partials      119      119           

Flag	Coverage Δ
integration	68.55% <ø> (ø)
unittests	92.11% <ø> (ø)

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Impacted Files	Coverage Δ
stonesoup/updater/chernoff.py	92.30% <ø> (ø)
stonesoup/updater/pointprocess.py	95.94% <ø> (ø)

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