AFT Survival Loss Scale Parameter is no longer in the docs

[bblodfon]

Hi,

I noticed that for AFT survival analysis there used to be a parameter called aft_loss_distribution_scale, but now it is not listed in the xgb.params() or in the online docs, any idea where did it go? :)

[trivialfis]

Thank you for raising the issue. The parameter is still here, but the document is missing.

cc @hcho3 when you are back