app: Improve wording on why repository housekeeping should stay enabled

What does this MR do and why?

The "Repository" admin area has a section that allows administrators to configure repository housekeeping. Next to some fine-grained settings, the admin may also choose to disable housekeeping entirely. The help text we have discourages this, but it does so without giving any reason whatsoever.

Reword the help text to point out what housekeeping does and why it should stay enabled: namely, to not regress performance.

Part of #365318 (closed).

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