Add docstrings to nidigital system tests that cover multiple methods

[ni-jfitzger]

• This contribution adheres to CONTRIBUTING.md.

• I've updated CHANGELOG.md if applicable.

• I've added tests applicable for this pull request

What does this Pull Request accomplish?

• Improves nidigital system test documentation of what methods are tested.

List issues fixed by this Pull Request below, if any.

What testing has been done?

Ran the system tests on a system with nidigital 19.0.1 installed.

[codecov]

Codecov Report

Merging #1428 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1428   +/-   ##
=======================================
  Coverage   91.90%   91.90%           
=======================================
  Files          20       20           
  Lines        3642     3642           
=======================================
  Hits         3347     3347           
  Misses        295      295           

Flag	Coverage Δ
#codegenunittests	88.24% <ø> (ø)
#nifakeunittests	96.36% <ø> (ø)
#nimodinstunittests	95.37% <ø> (ø)
#nitclkunittests	95.45% <ø> (ø)

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