installation: Clarify that SDKs aren't considered used

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The docs claimed that SDK runtimes are considered used even if there is
no app using them or runtime for which they are the SDK, but the
implementation does not match that.

We could change the implementation to always consider SDK runtimes used,
but that would be problematic because some apps use an SDK as their
runtime, and in that case the runtime would persist forever after the
app is uninstalled, instead of eventually being garbage collected by the
subsequent commits to this one.

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