cmd/go: update "go help doc" docs

This CL updates "go help doc" docs so they reflect the following
changes:

- CL 59413 modified "go doc", so the behavior of the two-args case is
  consistent with the one-arg one.
- CL 141397 removed godoc's command-line interface in favor of "go doc".

Fixes #49830.

Change-Id: I0923634291d34ae663fe2944d69757462b814919
Reviewed-on: https://go-review.googlesource.com/c/go/+/367497
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Trust: Russ Cox <[email protected]>

Author: jroimartin

src/cmd/go/alldocs.go

@@ -60,9 +60,8 @@ The package path must be either a qualified path or a proper suffix of a
 path. The go tool's usual package mechanism does not apply: package path
 elements like . and ... are not implemented by go doc.
 
-When run with two arguments, the first must be a full package path (not just a
-suffix), and the second is a symbol, or symbol with method or struct field.
-This is similar to the syntax accepted by godoc:
+When run with two arguments, the first is a package path (full path or suffix),
+and the second is a symbol, or symbol with method or struct field:
 
 	go doc <pkg> <sym>[.<methodOrField>]