help: revise explanation about capability check while selecting merge tool
This is follow up of 7c6044634957 and cded904f7acc.
This patch adds explanations about:
- notation in capability columns in the table
- how capabilities of external merge tools are treated
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)