math/rand: document Rand output compatibility promise

[dsnet]

Using go1.6

It seems from rand/regress_test.go that there is an explicit test to:

Test that random number sequences generated by a specific seed do not change from version to version.

The public documentation for math/rand doesn't really saying anything about this. If there is a hard guarantee for deterministic output of Rand between versions of Go, can this be documented?

[ianlancetaylor]

I don't think we want that to be a hard guarantee. There is some discussion in issue #8013. We don't want to change the sequence gratuitously, but we don't necessarily want to lock ourselves into that exact sequence if there is ever a really good reason to change it. The test is there to make sure that we don't change the sequence accidentally.