Document resumable downloads via `.part` files in `git lfs fetch`

While using git lfs fetch with the standard basic transfer adapter (against a Hugging Face LFS/Xet backend), I observed that interrupted downloads can be resumed if the partial download exists as:

.git/lfs/incomplete/<oid>.part

GIT_TRACE=1 shows:

xfer: Attempting to resume download of "<oid>" from byte N
HTTP: 206
xfer: server accepted resume download request

This behaviour appears to work reliably but I couldn't find it documented. The only references to resuming that I could find from a quick search of the repo were in docs/proposals/multipart_transfer_mode.md and a few in the t and tq paths (e.g. tq/basic_download.go)

If support for .part files an intentional feature of git-lfs, documenting it would be valuable for users downloading large LFS objects over unreliable connections. At present it seems that a manual file rename is required - renaming interrupted files in the incomplete path to the full oid and adding the part extension.

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