$ cd $GOROOT/src/os
$ go doc ./exec
package exec // import "os/exec"
$ go doc ./exec Cmd
doc: import "./exec": import relative to unknown directory
exit status 1
If cmd/doc can find the package docs on the first invocation, why couldn't it find the the same package on the 2nd invocation?
Additionally, the error also doesn't make sense: of course it knows the directory the import relative to (the cwd).