[fbgemm_gpu] Update build documentation

[q10]

• Update build documentation for folly

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@q10 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

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@q10 merged this pull request in pytorch/FBGEMM@fc212d4ff3faedf543e9625cfdc041bcbe5beb90.

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This pull request has been reverted by 3417e2d658b3ca4acc24a7cad62f626c02f3b07e.