expanded on explanation in note re: extends supported versions (#3696)

* expanded on explanation in note re: extends supported versions

Signed-off-by: Victoria Bialas <victoria.bialas@docker.com>

* copyedits

Signed-off-by: Victoria Bialas <victoria.bialas@docker.com>

* x-ref to open issue to bring back extends

Signed-off-by: Victoria Bialas <victoria.bialas@docker.com>

* added more x-refs to note on extends

Signed-off-by: Victoria Bialas <victoria.bialas@docker.com>

compose/compose-file/compose-versioning.md

@@ -205,7 +205,7 @@ Introduces the following additional parameters:
 ### Version 2.2
 
 An upgrade of [version 2.1](#version-21) that introduces new parameters only
-available with Docker Engine version **1.13.0+**.  Version 2.2 files are 
+available with Docker Engine version **1.13.0+**.  Version 2.2 files are
 supported by **Compose 1.13.0+**. This version also allows you to specify
 default scale numbers inside the service's configuration.
 
@@ -220,9 +220,10 @@ Designed to be cross-compatible between Compose and the Docker Engine's
 [swarm mode](/engine/swarm/index.md), version 3 removes several options and adds
 several more.
 
-- Removed: `volume_driver`, `volumes_from`, `cpu_shares`, `cpu_quota`, `cpuset`,
+- Removed: `volume_driver`, `volumes_from`, `cpu_shares`, `cpu_quota`,
-  `mem_limit`, `memswap_limit`, `extends`, `group_add`. See the [upgrading](#upgrading)
+`cpuset`, `mem_limit`, `memswap_limit`, `extends`, `group_add`. See
-  guide for how to migrate away from these.
+the [upgrading](#upgrading) guide for how to migrate away from these.
+(For more information on `extends`, please see [Extending services](/compose/extends.md#extending-services).)
 
 - Added: [deploy](index.md#deploy)
 
@@ -269,7 +270,8 @@ several options have been removed:
     `deploy`. Note that `deploy` configuration only takes effect when using
     `docker stack deploy`, and is ignored by `docker-compose`.
 
--   `extends`: This option has been removed for `version: "3.x"` Compose files.
+-   `extends`: This option has been removed for `version: "3.x"`
+Compose files. (For more information, please see [Extending services](/compose/extends.md#extending-services).)
 -   `group_add`: This option has been removed for `version: "3.x"` Compose files.
 -   `pids_limit`: This option has not been introduced in `version: "3.x"` Compose files.
 -   `link_local_ips` in `networks`: This option has not been introduced in

compose/extends.md

@@ -163,7 +163,17 @@ backup, include the `docker-compose.admin.yml` as well.
 
 ## Extending services
 
-> Up to version 2.1 , version 3.x does not support `extends` yet.
+> **Note**: The `extends` keyword is supported in earlier Compose file formats
+up to Compose file version 2.1 (see [extends in
+v1](/compose/compose-file/compose-file-v1.md#extends) and [extends in
+v2](/compose/compose-file/compose-file-v2.md#extends)), but is not supported in
+Compose version 3.x. See the [Version 3
+summary](/compose/compose-file/compose-versioning.md#version-3) of keys added
+and removed, along with information on [how to
+upgrade](/compose/compose-file/compose-versioning.md#upgrading). See
+[moby/moby#31101](https://github.com/moby/moby/issues/31101) to follow the
+discussion thread on possibility of adding support for `extends` in some form in
+future versions.
 
 Docker Compose's `extends` keyword enables sharing of common configurations
 among different files, or even different projects entirely. Extending services
@@ -171,12 +181,12 @@ 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`, `volumes_from`, and `depends_on` are never shared between
+Keep in mind that `links`, `volumes_from`, and `depends_on` are never shared
-> services using `extends`. These exceptions exist to avoid
+between services using `extends`. These exceptions exist to avoid implicit
-> implicit dependencies—you always define `links` and `volumes_from`
+dependencies; you always define `links` and `volumes_from` locally. This ensures
-> locally. This ensures dependencies between services are clearly visible when
+dependencies between services are clearly visible when reading the current file.
-> reading the current file. Defining these locally also ensures changes to the
+Defining these locally also ensures that changes to the referenced file don't
-> referenced file don't result in breakage.
+break anything.
 
 ### Understand the extends configuration
 
@@ -288,7 +298,9 @@ replaces the old value.
     # result
     command: python otherapp.py
 
-> **Note**: In the case of `build` and `image`, when using
+>  `build` and `image` in Compose file version 1
+>
+> In the case of `build` and `image`, when using
 > [version 1 of the Compose file format](compose-file.md#version-1), using one
 > option in the local service causes Compose to discard the other option if it
 > was defined in the original service.