Update merge docs with depends_on, and correction about how links and volumes_from are merged.

Signed-off-by: Daniel Nephin <dnephin@docker.com>

docs/extends.md

@@ -32,12 +32,9 @@ contains your base configuration. The override file, as its name implies, can
 contain configuration overrides for existing services or entirely new
 services.
 
-If a service is defined in both files, Compose merges the configurations using
-the same rules as the `extends` field (see [Adding and overriding
-configuration](#adding-and-overriding-configuration)), with one exception.  If a
-service contains `links` or `volumes_from` those fields are copied over and
-replace any values in the original service, in the same way single-valued fields
-are copied.
+If a service is defined in both files Compose merges the configurations using
+the rules described in [Adding and overriding
+configuration](#adding-and-overriding-configuration).
 
 To use multiple override files, or an override file with a different name, you
 can use the `-f` option to specify the list of files. Compose merges files in
@@ -176,10 +173,12 @@ is useful if you have several services that reuse a common set of configuration
 options. Using `extends` you can define a common set of service options in one
 place and refer to it from anywhere.
 
-> **Note:** `links` and `volumes_from` are never shared between services using
-> `extends`. See
-> [Adding and overriding configuration](#adding-and-overriding-configuration)
- > for more information.
+> **Note:** `links`, `volumes_from`, and `depends_on` are never shared between
+> services using >`extends`. These exceptions exist to avoid
+> implicit dependencies—you always define `links` and `volumes_from`
+> locally. This ensures dependencies between services are clearly visible when
+> reading the current file. Defining these locally also ensures changes to the
+> referenced file don't result in breakage.
 
 ### Understand the extends configuration
 
@@ -275,13 +274,7 @@ common configuration:
 
 ## Adding and overriding configuration
 
-Compose copies configurations from the original service over to the local one,
-**except** for `links` and `volumes_from`. These exceptions exist to avoid
-implicit dependencies—you always define `links` and `volumes_from`
-locally. This ensures dependencies between services are clearly visible when
-reading the current file. Defining these locally also ensures changes to the
-referenced file don't result in breakage.
-
+Compose copies configurations from the original service over to the local one.
 If a configuration option is defined in both the original service the local
 service, the local value *replaces* or *extends* the original value.
 

tests/unit/config/config_test.py

@@ -2313,7 +2313,7 @@ class ExtendsTest(unittest.TestCase):
         tmpdir = py.test.ensuretemp('test_extends_with_defined_version')
         self.addCleanup(tmpdir.remove)
         tmpdir.join('docker-compose.yml').write("""
-            version: 2
+            version: "2"
             services:
               base:
                 image: example