RFR: Improve the Download message in PR description

[Magnus Ihse Bursie]

On Fri, 12 Mar 2021 15:07:40 GMT, Erik Helin <ehelin at openjdk.org> wrote:

>> Both @simonis and I have struggled of finding the proper way to update a checked-out PR. Let's update the help text with Volker's findings of the best way to do this!
>
> You mean when you are updating your local copy of someone else's pull request? Because otherwise you just pull from your personal fork, right?

If you follow the instructions in the Download section to get a local copy of the PR, this works just fine. However, if you want to update your local copy when the PR has changed with new code, this turned out to be a non-trivial exercise in gitology for both me and Volker (and many more, I presume). I think this is understood from the context of the "Download" part.

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PR: https://git.openjdk.java.net/skara/pull/1053