Backport note about links being legacy (docker#4956)

Author: Misty Stanley-Jones

compose/compose-file/compose-file-v1.md

@@ -355,6 +355,9 @@ It's recommended that you use reverse-DNS notation to prevent your labels from c
 Link to containers in another service. Either specify both the service name and
 a link alias (`SERVICE:ALIAS`), or just the service name.
 
+> Links are a legacy option. We recommend using
+> [networks](#networks) instead.
+
     web:
       links:
        - db

compose/compose-file/compose-file-v2.md

@@ -702,6 +702,9 @@ It's recommended that you use reverse-DNS notation to prevent your labels from c
 Link to containers in another service. Either specify both the service name and
 a link alias (`"SERVICE:ALIAS"`), or just the service name.
 
+> Links are a legacy option. We recommend using
+> [networks](#networks) instead.
+
     web:
       links:
        - "db"

compose/compose-file/index.md

@@ -1007,7 +1007,7 @@ specifying both the container name and the link alias (`CONTAINER:ALIAS`).
 >
 > If you're using the [version 2 or above file format](compose-versioning.md#version-2), the externally-created  containers
 must be connected to at least one of the same networks as the service which is
-linking to them. Starting with Version 2, [links](compose-file-v2#links) are a
+linking to them. [Links](compose-file-v2#links) are a
 legacy option. We recommend using [networks](#networks) instead.
 >
 > This option is ignored when [deploying a stack in swarm mode](/engine/reference/commandline/stack_deploy.md)