Change wording of commit-map size advisory

What does this MR do?

The previous text suggested that the maximum size for a commit-map file should be 3000 lines, otherwise the background cleanup task may fail with timeout errors. This is very dependent on the capacity of the gitlab instance and resulted from a self-managed ticket. However in a recent ticket resizing a very complex repository on gitlab.com a much larger commit-map file was processed in one go without any problems, but only after the customer has spent many hours in an earlier attempt uploading 80+ 3000 line files due to following the documented advice, which may in itself have contributed to the process not working correctly.

So I've changed the wording to be less prescriptive and have increased the example line count to 20000.

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