Show documentation popup window even if an element has a marker

_Moved from [Eclipse Platform UI #3015](https://github.com/eclipse-platform/eclipse.platform.ui/issues/3015)_

## Problem

When a piece of code has a warning (see 1.), I still would like to be able to see the documentation in the popup window (like in 2.).

Pressing F2 normally opens the documentation popup of the element under the cursor, except when there is a marker attached to the element.

1:

2:

This would especially help in the pictured case of deprecation warnings where the information on what to use instead can be included in the JavaDoc of the deprecated method/class/etc.

IntelliJ has a neat solution for this: It displays both the warning message and the documentation in a combined popup window.


## Suggestion

1. Introduce a combined popup, similar to the one IntelliJ has.
2. Or at least add a keybind that makes it possible to open the documentation in a popup on elements with a warning.

## Community

- [x] I understand suggesting an enhancement doesn't mandate anyone to implement it. Other contributors may consider this suggestion, or not, at their own convenience. The most efficient way to get it fixed is that I implement it myself and contribute it back as a good quality patch to the project.

Start by tracing the F2 command that opens the documentation popup and how it handles elements with markers. Compare the warning and documentation cases shown in the issue, then determine whether the completed behavior should combine both messages or provide a separate way to open documentation when a warning is present.

Written by the indexing model from the issue text.