$ which go
/usr/local/go/bin/go
$ go version
go version go1.8.1 linux/amd64
$ go env GOROOT
/usr/local/go
$ godoc cmd/asm
2017/09/17 02:12:37 cmd/asm: no such directory or package
$ godoc asm
2017/09/17 02:12:39 open /usr/local/go/src/asm: no such file or directory
$ godoc cmd/gofmt
2017/09/17 02:12:45 cmd/gofmt: no such directory or package

			// Then try $GOROOT/cmd.
			abspath = pathpkg.Join(pres.CmdFSRoot(), path)
			cinfo = pres.GetCmdPageInfo(abspath, relpath, mode)

$ GOPATH=`pwd` go install golang.org/x/tools/cmd/godoc
$ ./bin/godoc cmd/asm
COMMAND DOCUMENTATION

    Asm, typically invoked as ``go tool asm'', assembles the source file
    into an object file named for the basename of the argument source file
    with a .o suffix. The object file can then be combined with other
    objects into a package archive.


    Command Line
<rest output>

$ ./bin/godoc asm 
<same output as above>

$ ./bin/godoc syscall | head
PACKAGE DOCUMENTATION

package syscall
    import "syscall"

    Package syscall contains an interface to the low-level operating system
    primitives. The details vary depending on the underlying system, and by
    default, godoc will display the syscall documentation for the current
    system. If you want godoc to display syscall documentation for another
    system, set $GOOS and $GOARCH to the desired system. For example, if you

../xtools/bin/godoc -goroot=`pwd` cmd/foo | head -n 4
2017/09/17 02:36:05 cmd/foo: no such directory or package

$godoc fmt | head
use 'godoc cmd/fmt' for documentation on the fmt command 

PACKAGE DOCUMENTATION

package fmt
    import "fmt"

    Package fmt implements formatted I/O with functions analogous to C's
    printf and scanf. The format 'verbs' are derived from C's but are
    simpler.

$godoc cmd/fmt | head
PACKAGE DOCUMENTATION

package fmt
    import "."

    Package fmt implements formatted I/O with functions analogous to C's
    printf and scanf. The format 'verbs' are derived from C's but are
    simpler.

...x/tools/godoc ((4b4231b9...) $) % cat ~/go/src/p1/p1.go
package p1
import "p2"

const V2 = p2.V
...x/tools/godoc ((4b4231b9...) $) % godoc p1
PACKAGE DOCUMENTATION

package p1
    import "p1"


CONSTANTS

const V2 = p2.V


...x/tools/godoc ((4b4231b9...) $) % godoc cmd/p1
2018/03/23 12:11:11 cmd/p1: no such directory or package
...x/tools/godoc ((4b4231b9...) $) % godoc syscall|head
PACKAGE DOCUMENTATION

package syscall
    import "syscall"

    Package syscall contains an interface to the low-level operating system
    primitives. The details vary depending on the underlying system, and by
    default, godoc will display the syscall documentation for the current
    system. If you want godoc to display syscall documentation for another
    system, set $GOOS and $GOARCH to the desired system. For example, if you
...x/tools/godoc ((4b4231b9...) $) % godoc cmd/syscall
2018/03/23 12:11:28 cmd/syscall: no such directory or package
...x/tools/godoc ((4b4231b9...) $) % godoc cmd/fmt|head
2018/03/23 12:11:34 cmd/fmt: no such directory or package
...x/tools/godoc ((4b4231b9...) $) % godoc cmd/gofmt|head
COMMAND DOCUMENTATION

    Gofmt formats Go programs. It uses tabs for indentation and blanks for
    alignment. Alignment assumes that an editor is using a fixed-width font.

    Without an explicit path, it processes the standard input. Given a file,
    it operates on that file; given a directory, it operates on all .go
    files in that directory, recursively. (Files starting with a period are
    ignored.) By default, gofmt prints the reformatted sources to standard
    output.