Document Bazel's terminal output, particularly action statistics

The Bazel terminal output reports statistics about build/test progress in a variety of ways that can be confusing for users. See, e.g., [this bazel-discuss thread](https://groups.google.com/forum/#!msg/bazel-discuss/9smBB-m5KOQ/GCfxJvcPBgAJ) for some examples. We should document what the statistics mean, along with pointers to the command line options that can be used to customize the output.

Start by reviewing Bazel's terminal output and the linked bazel-discuss thread to inventory the reported statistics and their meanings. Locate the relevant documentation and command-line options, then document the statistics and explain how users can customize the output; done means the confusing cases and customization pointers are covered.

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