fmt: explain how Formatter interface affects verbs and flags
Formatter is mentioned further down, but it's helpful
to add it amongst the verbs and flags.
Background: I spent a while puzzling how "%+v" prints
a stack trace for github.com/pkg/errors when this isn't
documented under 'flags'.
Change-Id: Ic70145902a36780147dedca568b3cf482974fc38
GitHub-Last-Rev:
6571b499f211a2266812af66dd3b88dff602cabf
GitHub-Pull-Request: golang/go#39860
Reviewed-on: https://go-review.googlesource.com/c/go/+/240000
Reviewed-by: Rob Pike <r@golang.org>
Trust: Rob Pike <r@golang.org>
Trust: Ian Lance Taylor <iant@golang.org>