Documentation on testing toolchain changes is out of date

@comius

This page is out of date: https://github.com/bazelbuild/java_tools/blob/master/docs/behind-the-release.md#manually-trying-out-java_tools-before-the-release-process

* The `//src:java_tools_javaX.zip` target doesn't exist anymore (I think it's been replaced by e.g. `//src:java_tools_zip`).

* It refers to `--java_toolchain`, which doesn't work with bazel at head due to the switch to toolchain resolution.