Update drop_if_exists docs to include constraints (#663)

Author: nkyllonen

lib/ecto/migration.ex

@@ -718,14 +718,19 @@ defmodule Ecto.Migration do
   end
 
   @doc """
-  Drops a table or index if it exists.
+  Drops one of the following if it exists:
+
+    * an index
+    * a table
+    * a constraint
 
   Does not raise an error if the specified table or index does not exist.
 
   ## Examples
 
       drop_if_exists index("posts", [:name])
       drop_if_exists table("posts")
+      drop_if_exists constraint("products", "price_must_be_positive")
       drop_if_exists index("posts", [:name]), mode: :cascade
       drop_if_exists table("posts"), mode: :cascade
 
@@ -736,12 +741,10 @@ defmodule Ecto.Migration do
       on the table. Default is `:restrict`
 
   """
-  def drop_if_exists(%{} = index_or_table, opts \\ []) when is_list(opts) do
-    Runner.execute(
-      {:drop_if_exists, __prefix__(index_or_table), Keyword.get(opts, :mode, :restrict)}
-    )
-
-    index_or_table
+  def drop_if_exists(%{} = index_or_table_or_constraint, opts \\ []) when is_list(opts) do
+    mode = Keyword.get(opts, :mode, :restrict)
+    Runner.execute({:drop_if_exists, __prefix__(index_or_table_or_constraint), mode})
+    index_or_table_or_constraint
   end
 
   @doc """