fmt: clarify wording of * flag

fmt: clarify wording of * flag

The complainant is confused by the ambiguity of 'next' in the
phrase 'next operand'. It seems clear enough to me that things
are always read left to right when formatting, but to calm the
waters we add a clarifying parenthetical.

Fixes #22275

Change-Id: I82418c1e987db736f4bee0faa53fe715c9cde8f5
Reviewed-on: https://go-review.googlesource.com/71010
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>