Resolving issue #1745 with Polymer docs

Polymer/old-docs-site#1745. Clarified info about deprecated combinators /deep/ and ::shadow

src/lib/custom-style.html

@@ -21,7 +21,7 @@
 
 * Document styles defined in a `custom-style` will be shimmed to ensure they do
 not leak into local DOM when running on browsers without non-native Shadow DOM.
-* Shadow DOM-specific `/deep/` and `::shadow` combinators will be shimmed on
+* For backwards compatibility, the deprecated shadow DOM-specific `/deep/` and `::shadow` combinators will be shimmed on
 browsers without non-native Shadow DOM.
 * Custom properties used by Polymer's shim for cross-scope styling
 may be defined in an `custom-style`.
@@ -45,11 +45,6 @@
       box-sizing: border-box;
     }
 
-    /* Can use /deep/ and ::shadow combinators */
-    body /deep/ .my-special-view::shadow #thing-inside {
-      background: yellow;
-    }
-
     /* Custom properties that inherit down the document tree may be defined */
     body {
       --my-toolbar-title-color: green;