update docs for genrule tool_files prop

According to go/roboleaf-busy-beavers-sandboxing, this field should just take in any data files needed by the tool. The existing description makes it sound like this property should only contain a single file that will be used as the tool itself.

Change-Id: I3ef3b8ceb52f7a7e6de9e0a897d5cc05c9c2d336