[cmd][diff] Ignore detection status for tags

[Szelethus]

We already do this on the GUI, but on the cmdline, we default the
detection status to NEW, REOPENED and UNRESOLVED. This makes sense for
runs (where we check whether a report is outstanding at the time of the
query), but not for tags (as a report can have a RESOLVED detection
status but still be outstanding at the time of the tag).

[bruntib]

Please, rebase. Otherwise looks good.

[dkrupp]

Please refer to this document from the usage guide.
Also please connect the sections to real PR analysis, branch analysis use-cases which should be introduced in the intro section. The user should understand how to use these concepts in practice.

[dkrupp]

Refer to the use cases introduced in the introduction section.

E.g.

If you whish to store the suppressions on the server, you can use review status rules. With the review status rule you can set all instances of report type (reports with the same hash) e.g. false positive. This feature makes it easy to mark a report false positive in all runs at the same time.

[Szelethus]

I don't see how this adds anything to the document. Also, I just don't think this is the place where we should properly document review status rules -- merely the place to discuss how their usage affects diff.

Also, I think the current shorthand explanation already does a decent job.

[dkrupp]

A report is outstanding in a given time T if all the following is true:

• if it was detected before T
• if it was closed after T, or not closed at all
• a report can be closed if becomes undetected, considered false positive, intentional bug.

The server stores the past states of a reports with certain simplifications (see section **), so it can tell for any past time points whether the report was outstanding or closed.

The local report directory only stores a single present state.

In CodeChecker the most recent status of a report is also represented in the detection status:
-new: the report is new compared to the last analysis
-reopened: the report is new compared to the last analysis and it is reappearing
-unresolved: the report is still detected
-resolved: the report is not detected anymore

This is useful to quickly see if for example the analysis set of the "master" branch is updated, what are the new/unresolved reports we still have to work with.

Please note however, the detection status only describe the present state and does not play a role when comparing any two (past) analysis result sets.

[Szelethus]

I added the "outstanding report" definition when we diff tags/timestamps.

I added links to the detection status and review status docs.

For the rest, this is just not the place to describe what detection statuses are, but rather their own page. On the fact that we ignore detection statuses for tags/timestamps there already is paragraph.

[dkrupp]

With the condition of fixin my comments in diff.md in #4006 (review)

[Szelethus]

I cherry-picked the docs changes into #4006, so it no longer has any dependencies.