Source upgrade doc: etcupdate no longer needs -B

[ziaee]

Summary

Note: We confusingly have the instructions to build the system in the
build manual, the UPDATING file, and the Makefile. These will get out
of sync and will be harmful when they do.

Fixes: ddf6fad0295a ("etcupdate: Make nobuild the default")

[des]

-B is still recommended on 15 and 14. I intended to merge the change, but there is no consensus around that yet.

[ziaee]

In D57644#1321727, @des wrote:

-B is still recommended on 15 and 14. I intended to merge the change, but there is no consensus around that yet.

Thus the fixes tag, to make sure they MFC (or not) together.

[des]

In D57644#1321744, @ziaee wrote:

Thus the fixes tag, to make sure they MFC (or not) together.

We have exactly zero tooling around the Fixes: tag. Your commit won't show up in my MFC reminders, and it won't show up in yours either unless you also add an MFC after: tag.

[ziaee]

In D57644#1321745, @des wrote:

In D57644#1321744, @ziaee wrote:

Thus the fixes tag, to make sure they MFC (or not) together.

We have exactly zero tooling around the Fixes: tag. Your commit won't show up in my MFC reminders, and it won't show up in yours either unless you also add an MFC after: tag.

Well. Yes, but... so? We still need to fix the doc bug introduced by the committed change. I'm not familiar with any better way to do it.

[ziaee]

I will commit this tonight if there are no objections or suggestions for improvement.