Starlark impl function naming convention not clear

It seems not consistent in a lot of places where `_impl` or `_impl` should be used as convention for the implementation function name. According to doc [here](https://docs.bazel.build/versions/master/skylark/rules.html#implementation-function), it should be `_impl`, but the [example](https://github.com/bazelbuild/examples/blob/master/rules/attributes/printer.bzl) given in the same section uses `_impl`. And the whole [doc](https://docs.bazel.build/versions/master/skylark/rules.html) uses a mix of the two. It caused confusion here https://github.com/bazelbuild/rules_docker/pull/724#discussion_r264527597.