bug#70459: Flymake small improvements

[Juri Linkov]

close 70459 30.0.50
stop

> 1. clicking on flymake fringe indicator will show buffer diagnostics;
> 2. display diagnostics buffers at the bottom;
> 3. to do the above there is a need to swap the order
>    of generating the buffer contents and displaying the buffer,
>    i.e. the same as was done recently in info.el for bug#70213.

This is pushed to master and closed.