[bzlmod] instructions following a pin operation are only for WORKSPACE

[alexeagle]

The instructions for bzlmod users are wrong. Somehow we should sense that we are running under bzlmod and print correct next steps.

$ bazel run @maven//:pin
INFO: Analyzed target @rules_jvm_external~4.5~maven~maven//:pin (67 packages loaded, 293 targets configured).
...
Next, please update your WORKSPACE file by adding the maven_install_json attribute and loading pinned_maven_install from @rules_jvm_external~4.5~maven~maven//:defs.bzl.

[vorburger]

Update https://github.com/bazelbuild/rules_jvm_external/blob/master/docs/bzlmod.md#installation when done.