Better document some combinations of flags

[antoyo]

Better document the different combinations of --release-sysroot and --release --sysroot and --release --release-sysroot.

Also explain what is the sysroot and why it is necessary.

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[FrancescoV1985]

Hello.
I would like to try to solve this issue.
Best Regards,
Francesco

[antoyo]

Thanks! I assigned the issue to you.

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