Docs for different types of MG Action Results on MG README.md

[plebhash]

The PR #1024 introduced a new MG Action Result. During review, @Fi3 suggested adding some docs on how and when to use this new Action Result.

I wanted to fulfill his request but I realized there's no docs for any of the multiple kinds of MG Action Results, so I realized this is an entirely separate task that deserves its own issue.

SV2 Action Results:

• MatchMessageType
• MatchMessageField
• GetMessageField
• MatchMessageLen
• MatchExtensionType
• CloseConnection
• SustainConnection
• None

SV1 Action Results:

• MatchMessageId
• MatchMessageField
• CloseConnection
• None

A new subsection should be added to the Actions section of MG README.md

[plebhash]

For adding extra context on the discussion that motivated this issue:

PR #1024 introduced the SustainConnection Action Result. It is a requirement for #1023, which aims to fix #912 .

Basically, we noticed that JDS was dropping the connection in a scenario where that kind of behavior is not desirable. Therefore, it is useful to get MG to make an assertion that is similar to CloseConnection, but the opposite: it asserts the connection was not dropped.

This Action Result should be used as following: the user should add it to the MG test whenever they want to assert that the connection did not drop after some specific Action. In case the connection does drop, MG panics and the test fails. If the connection sustains, MG tests go on.

[plebhash]

Just a reminder on another way MG docs could be improved. We could also have a new section on README.md that helps the reader interpret logs on MG tests.

It could be inspired by this comment where I clarified how to interpret logs for one specific MG test:

#928 (comment)