Document {:unsafe_fragment, fragment} option for :returning (#4711)

Add documentation for the new {:unsafe_fragment, fragment} tuple format
that can be passed to the :returning option. This allows raw SQL expressions
to be used in the RETURNING clause for PostgreSQL and OUTPUT clause for MSSQL.

Author: henryzhan013

lib/ecto/repo.ex

@@ -1655,6 +1655,8 @@ defmodule Ecto.Repo do
       returns all fields in the given schema. May be a list of
       fields, where a struct is still returned but only with the
       given fields. Or `false`, where nothing is returned (the default).
+      It also accepts `{:unsafe_fragment, fragment}` to pass a raw SQL
+      expression directly to the RETURNING clause (not escaped, use with caution).
       This option is not supported by all databases.
 
     * `:prefix` - The prefix to run the query on (such as the schema path
@@ -1822,8 +1824,10 @@ defmodule Ecto.Repo do
       of fields to be returned from the database. When `true`, returns
       all fields, including those marked as `load_in_query: false`. When
       `false`, no extra fields are returned. It will always include all
-      fields in `read_after_writes` as well as any autogenerated id. Be
-      aware that the fields returned from the database overwrite what was
+      fields in `read_after_writes` as well as any autogenerated id.
+      It also accepts `{:unsafe_fragment, fragment}` to pass a raw SQL
+      expression directly to the RETURNING clause (not escaped, use with caution).
+      Be aware that the fields returned from the database overwrite what was
       supplied by the user. Any field not returned by the database will be
       present with the original value supplied by the user. Not all databases
       support this option and it may not be available during upserts.
@@ -2040,10 +2044,12 @@ defmodule Ecto.Repo do
       of fields to be returned from the database. When `true`, returns
       all fields, including those marked as `load_in_query: false`. When
       `false`, no extra fields are returned. It will always include all
-      fields in `read_after_writes`. Be aware that the fields returned
-      from the database overwrite what was supplied by the user. Any field
-      not returned by the database will be present with the original value
-      supplied by the user. Not all databases support this option.
+      fields in `read_after_writes`. It also accepts `{:unsafe_fragment, fragment}`
+      to pass a raw SQL expression directly to the RETURNING clause (not escaped,
+      use with caution). Be aware that the fields returned from the database
+      overwrite what was supplied by the user. Any field not returned by the
+      database will be present with the original value supplied by the user.
+      Not all databases support this option.
 
     * `:force` - By default, if there are no changes in the changeset,
       `c:update/2` is a no-op. By setting this option to true, update
@@ -2158,10 +2164,12 @@ defmodule Ecto.Repo do
       of fields to be returned from the database. When `true`, returns
       all fields, including those marked as `load_in_query: false`. When
       `false`, no extra fields are returned. It will always include all
-      fields in `read_after_writes`. Be aware that the fields returned
-      from the database overwrite what was supplied by the user. Any field
-      not returned by the database will be present with the original value
-      supplied by the user. Not all databases support this option.
+      fields in `read_after_writes`. It also accepts `{:unsafe_fragment, fragment}`
+      to pass a raw SQL expression directly to the RETURNING clause (not escaped,
+      use with caution). Be aware that the fields returned from the database
+      overwrite what was supplied by the user. Any field not returned by the
+      database will be present with the original value supplied by the user.
+      Not all databases support this option.
 
     * `:prefix` - The prefix to run the query on (such as the schema path
       in Postgres or the database in MySQL). This overrides the prefix set