cmd/doc: add option to output a clean one-line symbol representation

Currently there is no way for go doc to output a clean
one-line symbol representation of types, functions, vars
and consts without documentation lines or other text lines
added.

For example `go doc fmt` has a huge introduction so if you
pass that to grep or fzf to search a symbol let say scan
`go doc fmt | grep scan` you get way to many false
positives.

Added a `-short` flag to be able to do
`go doc -short fmt | grep scan` instead which will result in
just the symbols you are looking for.

func Fscan(r io.Reader, a ...interface{}) (n int, err error)
func Fscanf(r io.Reader, format string, a ...interface{}) (n int, err error)
func Fscanln(r io.Reader, a ...interface{}) (n int, err error)
func Sscan(str string, a ...interface{}) (n int, err error)
func Sscanf(str string, format string, a ...interface{}) (n int, err error)
func Sscanln(str string, a ...interface{}) (n int, err error)


Fixes #32597

Change-Id: I77a73838adc512c8d1490f5a82075de6b0462a31
Reviewed-on: https://go-review.googlesource.com/c/go/+/184017


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>