Several things to be covered here:
The ‘reviewed’ option is a setting specifically affecting what will be displayed for you in the Identify module. Everything that is marked as ‘reviewed’ will not be displayed for you by the default settings (which can be modified, however).
In other words: The ‘reviewed’ button only changes the display for your personal account, it won’t have any effect on the observation for any other user.
So, I don’t think it works as you assume, regarding the dashboard:
But I stand corrected.
However,
- this should not be possible, as when you leave an ID on someone’s observation, the ‘reviewed’ box should automatically be ticked, so you would only be able to de-select it again. Just viewing or commenting on someone’s observation won’t trigger the ‘reviewed’ field.
Next topic:
If you don’t want to receive notifications about agreeing ID’s, you can change your personal settings accordingly, by turning Confirming IDs off:
You will still be notified if someone disagrees with your ID.
You can also stop following observations on a case-by-case level by choosing ‘unfollow’ in the upper right corner on an observation page:
or likewise in the Identify module:
I’d assume that in ‘unfollowing’ an observation you also won’t be notified about disagreeing IDs, however.
Lastly,
I feel the help section would become too large and hard to view if every detail would be covered there.
But often, you can find further information about a function by either hovering your mouse over it or clicking on it (or on a ‘?’ or ‘i’ beside of it)
E.g., clicking:
on the ‘i’
on the ‘?’
even two, redundant, options here (either ‘what’s this’ or ‘about’)
Or, via mouse-over:
the annotations are further explained that way
quality grades on an observation are also explained via this method
And also the ‘reviewed’ function is explained that way:

Whether this information is easy to find, I am not commenting on.
But I wanted to show that at least sometimes, explanations exist.