doc: documented new Docker code

src/util/util.v

@@ -93,6 +93,9 @@ pub fn pretty_bytes(bytes int) string {
 	return '${n:.2}${util.prefixes[i]}'
 }
 
+// match_array_in_array<T> returns how many elements of a2 overlap with a1. For
+// example, if a1 = "abcd" & a2 = "cd", the result will be 2. If the match is
+// not at the end of a1, the result is 0.
 pub fn match_array_in_array<T>(a1 []T, a2 []T) int {
 	mut i := 0
 	mut match_len := 0