cmd/doc: add -src flag

[adg]

The godoc command has a -src flag that prints the source of the specified identifier.

It would be great if the doc tool had this feature, too, so that one could do this:

$ go doc -src io.readfull
// ReadFull reads exactly len(buf) bytes from r into buf.
// It returns the number of bytes copied and an error if fewer bytes were read.
// The error is EOF only if no bytes were read.
// If an EOF happens after reading some but not all the bytes,
// ReadFull returns ErrUnexpectedEOF.
// On return, n == len(buf) if and only if err == nil.
func ReadFull(r Reader, buf []byte) (n int, err error) {
    return ReadAtLeast(r, buf, len(buf))
}

[adg]

cc @robpike

[gopherbot]

Change https://golang.org/cl/140959 mentions this issue: cmd/doc: add a -src flag to show original source