This text is added to doc.go:

        Explicit argument indexes:

        In Printf, Sprintf, and Fprintf, the default behavior is for each
        formatting verb to format successive arguments passed in the call.
        However, the notation [n] immediately before the verb indicates that the
        nth one-indexed argument is to be formatted instead. The same notation
        before a '*' for a width or precision selects the argument index holding
        the value. After processing a bracketed expression [n], arguments n+1,
        n+2, etc. will be processed unless otherwise directed.

        For example,
                fmt.Sprintf("%[2]d %[1]d\n", 11, 22)
        will yield "22, 11", while
                fmt.Sprintf("%[3]*[2].*[1]f", 12.0, 2, 6),
        equivalent to
                fmt.Sprintf("%6.2f", 12.0),
        will yield " 12.00". Because an explicit index affects subsequent verbs,
        this notation can be used to print the same values multiple times
        by resetting the index for the first argument to be repeated:
                fmt.Sprintf("%d %d %#[1]x %#x", 16, 17)
        will yield "16 17 0x10 0x11".

The notation chosen differs from that in C, but I believe it's easier to read
and to remember (we're indexing the arguments), and compatibility with
C's printf was never a strong goal anyway.

While we're here, change the word "field" to "arg" or "argument" in the
code; it was being misused and was confusing.

R=rsc, bradfitz, rogpeppe, minux.ma, peter.armitage
CC=golang-dev
https://golang.org/cl/9680043