What's the behavior when go_proto_library's 'importpath' differ with 'option go_package'?

### What version of rules_go are you using?
Mainline

### What version of gazelle are you using?
Newest.

### What version of Bazel are you using?
Build label: 0.23.0rc1
Build target: bazel-out/k8-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar
Build time: Wed Feb 6 12:12:32 2019 (1549455152)
Build timestamp: 1549455152
Build timestamp as int: 1549455152

### Does this issue reproduce with the latest releases of all the above?
Not relevant.

### What operating system and processor architecture are you using?
uname -r: 4.15.0-1035-oem

cat /etc/os-release:
NAME="Ubuntu"
VERSION="18.04.2 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.2 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

### Any other potentially useful information about your toolchain?
N/A

### What did you do?
N/A

### What did you expect to see?
Documentation get updated.

### What did you see instead?
Documentation does not mention this.

Start by locating the documentation for go_proto_library and comparing the documented meaning of 'importpath' with the protobuf option go_package. Confirm and document the behavior when they differ; done when the relevant documentation explicitly explains this interaction.

Written by the indexing model from the issue text.