Add documentation regarding environment variables

[mslipper]

Closes #1973.

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added entries in PENDING.md with issue #

• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #2171 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #2171   +/-   ##
========================================
  Coverage    63.91%   63.91%           
========================================
  Files          134      134           
  Lines         8194     8194           
========================================
  Hits          5237     5237           
  Misses        2604     2604           
  Partials       353      353

[alexanderbez]

Looks like this was already addressed in #2170. Should not have it in two PRs I think.

[mslipper]

@alexanderbez I will remove from the other PR.

[ValarDragon]

LGTM, left a comment on the other PR to remove it from there.

[cwgoes]

utACK