Some of the information here: https://www.openembedded.org/pipermail/bitbake-devel/2017-May/008640.html should be added to the bitbake manual in the git fetcher section.
Patch sent to the docs list: https://lists.yoctoproject.org/g/docs/topic/patch_ref_manual_document/84865622
Adopting this bug, not so much left to complete it (send it to the right list for inclusion in BitBake documentation)
Patch updated according to feedback and sent to the bitbake-devel mailing list: https://lists.openembedded.org/g/bitbake-devel/message/12811 https://lists.openembedded.org/g/bitbake-devel/message/12812
Medium+ issues should pick a milestone so I picked M1 arbitrarily Michael.
This variable is now described in the BitBake manual. Closing the bug.