clarify filter usage

The current paragraph is not complete, since a digit is not a special character.

Changing it to:

    If the key contains special characters or starts with a digit,
    you need to surround it with double quotes like this:
    `."foo$"`, or else `.["foo$"]`.

docs/content/manual/v1.5/manual.yml

@@ -278,8 +278,8 @@ sections:
           JSON object (aka dictionary or hash) as input, it produces
           the value at the key "foo", or null if there's none present.
 
-          If the key contains special characters, you need to surround
-          it with double quotes like this: `."foo$"`.
+          If the key contains special characters or starts with a digit,
+          you need to surround it with double quotes like this: `."foo$"`.
 
           A filter of the form `.foo.bar` is equivalent to `.foo|.bar`.