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
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL