cmd/doc: delete

Users should not invoke "go tool doc" directly, but should instead run
"go doc". Remove "cmd/doc" because it no longer invokes the "doc" tool
but incorporates its logic directly.

Fixes #74667

Change-Id: I357a3d7e0ca075f028df66e34951a41354c08941
Reviewed-on: https://go-review.googlesource.com/c/go/+/689835
Reviewed-by: Michael Matloob <[email protected]>
Auto-Submit: Michael Matloob <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: matloob

doc/next/3-tools.md

@@ -2,5 +2,10 @@
 
 ### Go command {#go-command}
 
+<!-- go.dev/issue/74667 -->
+`cmd/doc`, and `go tool doc` have been deleted. `go doc` can be used as
+a replacement for `go tool doc`: it takes the same flags and arguments and
+has the same behavior.
+
 ### Cgo {#cgo}