docs: update documentation around how grouping processes and tree mode are incompatible

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

Clarify some behaviour.

Issue

If applicable, what issue does this address?

Closes: #1677

Testing

If relevant, please state how this was tested. All changes must be tested to work:

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.02%. Comparing base (393c24d) to head (1de2501).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1679   +/-   ##
=======================================
  Coverage   42.02%   42.02%           
=======================================
  Files         110      110           
  Lines       17503    17503           
=======================================
  Hits         7356     7356           
  Misses      10147    10147           

Flag	Coverage Δ
macos-14	37.69% <ø> (ø)
ubuntu-latest	43.83% <ø> (ø)
windows-2019	37.59% <ø> (ø)

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