cmd/doc: correctly indent pre-formatted blocks

cmd/doc: correctly indent pre-formatted blocks

They were previously indented at the same level as the normal text when
printing a single symbol or the description of a field.

Running "go doc text/template Must":
Before:
    func Must(t *Template, err error) *Template
        Must is a helper that wraps a call to a function returning (*Template,
        error) and panics if the error is non-nil. It is intended for use in
        variable initializations such as

        var t = template.Must(template.New("name").Parse("text"))

After:
    func Must(t *Template, err error) *Template
        Must is a helper that wraps a call to a function returning (*Template,
        error) and panics if the error is non-nil. It is intended for use in
        variable initializations such as

            var t = template.Must(template.New("name").Parse("text"))

Running "go doc http Request.Header":
Before:
    type Request struct {
        // Header contains the request header fields either received
        // by the server or to be sent by the client.
        //
        // If a server received a request with header lines,
        //
        // Host: example.com
        // accept-encoding: gzip, deflate
        // Accept-Language: en-us
        // fOO: Bar
        // foo: two
        //
        // then
        //
        // Header = map[string][]string{
        // "Accept-Encoding": {"gzip, deflate"},
        // "Accept-Language": {"en-us"},
        // "Foo": {"Bar", "two"},
        // }
        ...

After:
    type Request struct {
        // Header contains the request header fields either received by the server or
        // to be sent by the client.
        //
        // If a server received a request with header lines,
        //
        //     Host: example.com
        //     accept-encoding: gzip, deflate
        //     Accept-Language: en-us
        //     fOO: Bar
        //     foo: two
        //
        // then
        //
        //     Header = map[string][]string{
        //          "Accept-Encoding": {"gzip, deflate"},
        //          "Accept-Language": {"en-us"},
        //          "Foo": {"Bar", "two"},
        //     }
        ...

Fixes #29708

Change-Id: Ibe1a6a7a76d6b19c5737ba6e8210e3ad0b88ce16
GitHub-Last-Rev: 439c0fe70a01490cbd9c3613eba3fe45a3ffd9be
GitHub-Pull-Request: golang/go#31120
Reviewed-on: https://go-review.googlesource.com/c/go/+/169957
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>