Update all affected PR/MR descriptions when creating or retargeting a PR/MR

[PawelLipski]

https://github.com/VirtusLab/git-machete/issues/1322#issuecomment-2347003840:

One nice addition would be to update other MR's when creating or retargeting one. Right now if I add a downstream MR it seems i have to go update all the other ones in the stack to get their descriptions to show it.

There's a related feature in the working queue: https://github.com/VirtusLab/git-machete/issues/1215, but this is about retargeting PRs and updating their description as traverse walks the branches. Still, a flag like --update-all-descriptions to {create,restack,retarget}-{mr,pr} would be useful.