update docs, minor unit test change

Author: milmazz

integration_test/pg/migrations_test.exs

@@ -153,7 +153,6 @@ defmodule Ecto.Integration.MigrationsTest do
       assert up_log =~ ~s(DROP CONSTRAINT "my_comments_user_id_fkey",)
       refute up_log =~ ~s(ALTER COLUMN "user_id" TYPE)
       assert up_log =~ ~s/ADD CONSTRAINT "my_comments_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "my_users"("id") ON DELETE SET NULL/
-      assert up_log =~ ~s{ALTER TABLE "my_comments" DROP CONSTRAINT "my_comments_user_id_fkey", ADD CONSTRAINT "my_comments_user_id_fkey" FOREIGN KEY ("user_id") REFERENCES "my_users"("id") ON DELETE SET NULL}
 
       assert up_log =~ ~s(ALTER TABLE "my_posts")
       refute up_log =~ ~s(ALTER COLUMN "user_id" TYPE)

lib/ecto/migration.ex

@@ -1283,10 +1283,9 @@ defmodule Ecto.Migration do
   > type update can lock the table, even if the type actually
   > doesn't change.
   >
-  > We have considered changing the column type even when it is not needed
-  > could lead to undesirable locks, that's why, at least in the PostgreSQL
-  > adapter, if you provide the option `:from`, and the column type matches,
-  > we will skip updating it.
+  > These undesired locks can be avoided when using the PostgreSQL adapter by
+  > providing the `:from` option and ensuring its type matches the type provided
+  > to `modify/3`. In that scenario, the type change part of the migration is omitted.
   >
   > Examples:
   >