keepassxc: note the native messaging hosts manifest installation conflict

[bmrips]

Description

Mention the problems that can occur when enabling the KeePassXC module and its possible fixes.

Closes #7298.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

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  {long description}
  

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  • Basic tests added. See Tests

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