(doc) Add information about environment variable

[gep13]

Description Of Changes

This commit addresses that, by adding it into the correct location.

Motivation and Context

When we shipped CLI 2.5.0, we added a new environment variable called ChocolateyPreviousPackageVersion, which stores the version number for the previously installed package version, and this is set when doing a "choco upgrade". However, this environment variable never made its way into the docs site.

Testing

• I have previewed these changes using the Docker Container or another method before submitting this pull request.

Change Types Made

• Minor documentation fix (typos etc.).
• Major documentation change (refactoring, reformatting or adding documentation to existing page).
• New documentation page added.
• The change I have made should have a video added, and I have raised an issue for this.
  • Issue #

Change Checklist

• Requires a change to menu structure (top or left-hand side)/
• Menu structure has been updated
• Images added to the img repository?
  • PR -

Related Issue

N/A

[AdmiringWorm]

Just a single question, otherwise this looks fine.

[AdmiringWorm]

LGTM