       the optional dependency on its own, the requirements and options
       specified here will also be applied.
 
-    * `:only` - the dependency will belong only to the given environments,
-      useful when declaring dev- or test-only dependencies
+    * `:only` - the dependency is made available only in the given environments,
+      useful when declaring dev- or test-only dependencies; by default the
+      dependency will be available in all environments. The value of this option
+      can either be a single environment (like `:dev`) or a list of environments
+      (like `[:dev, :test]`)
 
     * `:override` - if set to `true` the dependency will override any other
       definitions of itself by other dependencies

    * `:sparse`     - checkout a single directory inside the git repository and use it
      as your Mix dependency. Search "sparse git checkouts" for more information.

  ## Path options (`:path`)

    * `:path`        - the path for the dependency
    * `:in_umbrella` - when `true`, sets a path dependency pointing to

      try to infer the type of project but it can be overridden with this
      option by setting it to `:mix`, `:rebar`, `:rebar3` or `:make`

  ### Git options (`:git`)

    * `:git`        - the Git repository URI
    * `:github`     - a shortcut for specifying Git repos from GitHub, uses `git:`
