The documentation of upvar still contains the phrase:
Level may have any of the forms permitted for the uplevel command, and may be omitted if the first letter of the first otherVar is not # or a digit (it defaults to 1).
After the changes in bug 2673163 the level may always be omitted. So the old phrasing is confusing and not needed. I suggest to change it to:
If level is omitted, it defaults to 1.