Added note for session renewal error in mock server

[sanchi-t]

Proposed changes

This PR addresses Issue: [Link to Github issue within https://github.com/zowe/zss/issues if any]

This PR depends upon the following PRs:

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Change in a documentation
• Refactor the code
• Chore, repository cleanup, updates the dependencies.
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

PR Checklist

Please delete options that are not relevant.

• If the changes in this PR are meant for the next release / mainline, this PR targets the "staging" branch.
• My code follows the style guidelines of this project (see: Contributing guideline)
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes
• video or image is included if visual changes are made
• Relevant update to CHANGELOG.md
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works, or describe a test method below

Testing

Further comments

[DivergentEuropeans]

Can you please add that this is specifically in combined use with the App server + Zowe Desktop. ZSS is just an agent, an in theory (architecture) could be used elsewhere

[sanchi-t]

Sure