rename resultcallback to result_callback

[davidism]

This is a clearer name for the decorator. The attribute is renamed to _result_callback, the only mention in the docs already implied that it shouldn't be used directly and should be set with the parameter or decorator.

• fixes Docs: MultiCommand.resultcallback vs MultiCommand.result_callback #1160

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.