improve doc comments

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

internal/file/opener.go

@@ -5,10 +5,12 @@ import (
 	"os"
 )
 
+// Opener is an object that stores a path to later be opened as a file.
 type Opener struct {
 	path string
 }
 
+// Open the stored path as a io.ReadCloser.
 func (o Opener) Open() (io.ReadCloser, error) {
 	return os.Open(o.path)
 }