Make doc regarding masked indices more clear
See this [issue](https://github.com/huggingface/transformers/issues/2418) for details, basically there used to be different ways of specifying masked indices (either -1 or -100), which was fixed by this [commit](https://github.com/huggingface/transformers/commit/418589244d263087f1d48655f621a65f2a5fcba6).

However the doc remains unclear, this PR fixes this.

I had an issue originally because I was using a version which did not incorporate the uniformisation yet.
Fantastic, thanks @r0mainK!