Warning message on PR descriptions needs an update

[FabriceCastel]

It seems that it is in fact FINE to:

• merge the lowest PR in the stack through the GitHub UI
• run git spr update
• proceed as usual

I assume the way things can go "bad" is if you accidentally merge a branch that's targeting one of the automatically created spr branches instead of main, for example.

The error message that gets tacked on at the end of PR messages should be updated to reflect this. Either of the following seems reasonable to me:

• omit the message entirely for the lowest PR in the stack
• update the message to explain what github UI merge opterations do or do not break the expected flow with spr