Update docs for 1.2.0

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

docs/yml.md

@@ -173,8 +173,12 @@ env_file:
   - /opt/secrets.env
 ```
 
+Compose expects each line in an env file to be in `VAR=VAL` format. Lines
+beginning with `#` (i.e. comments) are ignored, as are blank lines.
+
 ```
-RACK_ENV: development
+# Set Rails/Rack environment
+RACK_ENV=development
 ```
 
 ### extends
@@ -217,42 +221,10 @@ Here, the `web` service in **development.yml** inherits the configuration of
 the `webapp` service in **common.yml** - the `build` and `environment` keys -
 and adds `ports` and `links` configuration. It overrides one of the defined
 environment variables (DEBUG) with a new value, and the other one
-(SEND_EMAILS) is left untouched. It's exactly as if you defined `web` like
-this:
+(SEND_EMAILS) is left untouched.
 
-```yaml
-web:
-  build: ./webapp
-  ports:
-    - "8000:8000"
-  links:
-    - db
-  environment:
-    - DEBUG=true
-    - SEND_EMAILS=false
-```
-
-The `extends` option is great for sharing configuration between different
-apps, or for configuring the same app differently for different environments.
-You could write a new file for a staging environment, **staging.yml**, which
-binds to a different port and doesn't turn on debugging:
-
-```
-web:
-  extends:
-    file: common.yml
-    service: webapp
-  ports:
-    - "80:8000"
-  links:
-    - db
-db:
-  image: postgres
-```
-
-> **Note:** When you extend a service, `links` and `volumes_from`
-> configuration options are **not** inherited - you will have to define
-> those manually each time you extend it.
+For more on `extends`, see the [tutorial](extends.md#example) and
+[reference](extends.md#reference).
 
 ### net
 
@@ -264,6 +236,16 @@ net: "none"
 net: "container:[name or id]"
 net: "host"
 ```
+### pid
+
+```
+pid: "host"
+```
+
+Sets the PID mode to the host PID mode.  This turns on sharing between
+container and the host operating system the PID address space.  Containers
+launched with this flag will be able to access and manipulate other
+containers in the bare-metal machine's namespace and vise-versa.
 
 ### dns