Mention TF_FORCE_GPU_ALLOW_GROWTH in using_gpu.md
Mention TF_FORCE_GPU_ALLOW_GROWTH in using_gpu.md

As described in [this comment](https://github.com/tensorflow/tensorflow/commit/5d2047029a77545c97c0fdf74d9c03c92d1dcb88#commitcomment-33445269) to the commit of discussed functionality, I'd like to have it mentioned here, as it is the most relevant place for it to be mentioned that I could find.

As I failed to find info about whether this feature is intended to be kept around and integrated into TF2 in the future development or its future is not that bright, I'd like to at least start a discussion about it by this PR and maybe advocate for it, as I find it most useful and would really appreciate for it to be standardized/propagated into future TF2 stable releases.
tensorflow/docs,tensorflow/docs,tensorflow/docs