[build] Bazel: JNI logic is unclear

[Gold856]

The Bazel logic to build JNI code is unclear. The reasoning behind the use of the zipper tool is unclear, and some of the compilation logic is also a little hard to follow. What's the significance of the return type of _jni_headers_impl? What's cc_common.merge_cc_infos? That's completely undocumented in Bazel docs. Some comments explaining what the different function calls do would be nice, or if there's good documentation online, some links would also be nice.