The out argument of a genrule does not have to be unique
The out argument of a genrule does not have to be unique

Summary:
- Removed text that says `out` parameter of `genrule` (and its relatives) must be unique.
- Clarified that the value provided for `out` is prop'd to the shell command in the `OUT` environment variable.

Reviewed By: cjhopman

fbshipit-source-id: af29871

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