Sanity bot should provide more hints for new users

Currently we have, for example:

• "Bug reference in summary" (e.g. https://codereview.qt-project.org/207371)

• "2nd line is not empty"

• "URL pointing to Gerrit or JIRA" (both https://codereview.qt-project.org/206650)

where new contributors are confronted with the sanity bot's error message and may feel a bit lost. As a reviewer, I have extra work explaining them how to fix the message.

The bot could already contain some hints or provide a link to a website with more explanations.

E.g.: for "2nd line not empty":

Patch summary in one short line followed by an empty line

Detailed patch description follows separated by an empty line.
This can stretch multiple lines and often contains [ChangeLog]
entries.

or "URL pointing to Gerrit or JIRA":

Patch summary in one short line

Please do not use links to JIRA in commit messages. The bug is 
referenced by a "Task-number" line at the commit message bottom, 
directly above the Change-Id line:

Task-number: QTBUG-12345
Change-Id: xyz