Document need for short paths when building on Windows

From build-dev:

Hello,
This is a followup of
https://mail.openjdk.java.net/pipermail/build-dev/2021-January/029905.html.

In that thread, my build on Windows 10 (not WSL) failed because Windows did
not short path some directories with space in it, such as "Microsoft Visual
Studio" or "Microsoft SDKs", while fixpath.sh expects Windows to do so.
This can be resolved by using fsutil to create short paths for them.

A direct consequence is that configure emits an uninformative message of
"Cannot find a valid Visual Studio installation" because all the paths it
received from fixpath.sh is "".

I believe it would be helpful if we add information on this problem in the
building instructions. Would anyone support this idea and possibly create a
ticket on openjdk bug tracker?

Best,
liangchenblue@gmail.com