Help: Clarify TARGET_RUNTIME_DLLS behavior on imported targets

This generator expression does not report the locations of `.dll`
files on imported targets with the `UNKNWON` type, since their
`IMPORTED_LOCATION` refers to the import library and not the DLL.

Fixes: #22845
50 jobs for !6700 with doc-TARGET_RUNTIME_DLLS in 7 minutes and 42 seconds (queued for 2 seconds)
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