docs: clarify descriptions for nix-env --delete-generations time and count

[eiis1000]

Closes #15112 .

Motivation

The page following page: https://nix.dev/manual/nix/2.28/command-ref/nix-env/delete-generations.html#generations-count
Says:

+<number>

The last number generations up to the present

Example: +5

Keep the last number generations, along with any newer than current.

While the last sentence is clear, the first one could be misconstrued to mean "delete the most recent generations" (useful for e.g. "oops my previous four generations had an important mistake, I want to delete them); somebody with this interpretation might accidentally delete all old generations EXCEPT for the broken ones. (Ask me how I know...)

This PR makes that description mildly less confusing, and also updates the description for the d option in the same vein.

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