Clarify that NativeLibrary can be used for transitive dependencies

[sweemer]

I have added the following two clarifications:

1. NativeLibrary can be used more than once. Originally I thought that it can only be used for the library specified in DirectPInvoke, but after some time I realized it can be used for transitive dependencies as well. My change makes this more clear.
2. Added /path/to to make it clear that paths are accepted and that the file does not need to be manually moved to the project output directory in order to be found.

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