committers guide: mention that multiple PRs may be specified on the same line

[emaste]

Summary

The PR updater script accepts comma- and space-separated PR lists, and svnweb is not making PR links even for single PRs. Many commits already specify multiple PRs on one line and it's a nicer (more compact) format, so document that.

[bcr]

Thanks for making the change, looks good to me.

[emaste]

rD50230