Document configuration transitions

[laurentlb]

Documentation of the cfg argument in rule is unclear (https://docs.bazel.build/versions/0.25.0/skylark/lib/globals.html#rule):

If set, points to the configuration transition the rule will apply to its own configuration before analysis.

I don't think a user will be able to understand and use that.

Rules documentation doesn't mention it either (https://docs.bazel.build/versions/master/skylark/rules.html). The Example repository (https://github.com/bazelbuild/examples/tree/master/rules) also doesn't help.

[juliexxia]

(Context: I'm moving on from Bazel on friday and spending some time triaging all bugs assigned to me before unassigning myself)

This is still a confusing thing. cfg should be documented for both the rule() function (which can only be a starlark transition) and the attr.label/_list() functions (which can be an unfortunately wide variety of things).

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