Merge pull request compose-spec#281 from epuronta/secret-target-tweak

Clarify secret.target absolute path syntax

spec.md

@@ -1645,7 +1645,7 @@ the service's containers.
 
 - `source`: The name of the secret as it exists on the platform.
 - `target`: The name of the file to be mounted in `/run/secrets/` in the
-  service's task containers. Defaults to `source` if not specified.
+  service's task container, or absolute path of the file if an alternate location is required. Defaults to `source` if not specified.
 - `uid` and `gid`: The numeric UID or GID that owns the file within
   `/run/secrets/` in the service's task containers. Default value is USER running container.
 - `mode`: The [permissions](http://permissions-calculator.org/) for the file to be mounted in `/run/secrets/`