Support :GoDoc with only package name

This change jumps to the top of the help pane when :GoDoc is run with only a package name.

Currently, `:GoDoc os` will scroll to `func (f *File) Chdir() error`, because the first search clause expands to `func (` (i.e., the first method definition). The proposed change always displays the top-level package documentation when only given a package name. This also maintains parity with `misc/vim`'s Godoc command.