Document that (and why) we disable MoveIt trajectory execution monitoring

[fmauch]

Feature summary

In #1132 we disabled trajectory execution monitoring (TEM) because it doesn't make much sense to use that with the scaled JTC.

However, as pointed out in #1132 (comment) this might lead to unexpected behavior for users that are familiar with TEM as this basically shifts the responsibility to monitor that to the user (the action client).

I think, we should at least add that to the usage section and the package doc.