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/manual.yml

@@ -317,8 +317,9 @@ sections:
           is, keys that are all made of alphanumeric characters and
           underscore, and which do not start with a digit.
 
-          If the key contains special characters, you need to surround
+          If the key contains special characters or starts with a digit,
-          it with double quotes like this: `."foo$"`, or else `.["foo$"]`.
+          you need to surround it with double quotes like this:
+          `."foo$"`, or else `.["foo$"]`.
 
           For example `.["foo::bar"]` and `.["foo.bar"]` work while
           `.foo::bar` does not, and `.foo.bar` means `.["foo"].["bar"]`.

docs/content/manual/v1.4/manual.yml

@@ -212,8 +212,8 @@ 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
+          If the key contains special characters or starts with a digit,
-          it with double quotes like this: `."foo$"`.
+          you need to surround it with double quotes like this: `."foo$"`.
 
           A filter of the form `.foo.bar` is equivalent to `.foo|.bar`.
 

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
+          If the key contains special characters or starts with a digit,
-          it with double quotes like this: `."foo$"`.
+          you need to surround it with double quotes like this: `."foo$"`.
 
           A filter of the form `.foo.bar` is equivalent to `.foo|.bar`.
 

docs/content/manual/v1.6/manual.yml

@@ -310,8 +310,9 @@ sections:
           is, keys that are all made of alphanumeric characters and
           underscore, and which do not start with a digit.
 
-          If the key contains special characters, you need to surround
+          If the key contains special characters or starts with a digit,
-          it with double quotes like this: `."foo$"`, or else `.["foo$"]`.
+          you need to surround it with double quotes like this:
+          `."foo$"`, or else `.["foo$"]`.
 
           For example `.["foo::bar"]` and `.["foo.bar"]` work while
           `.foo::bar` does not, and `.foo.bar` means `.["foo"].["bar"]`.