Clarify rules for package name in documentation

Nowhere in the documentation does it say that the only valid package names
are ones that match (excuse my abuse of regex to explain this):

^[A-Za-z][A-Za-z0-9_]

This change makes clear the rules followed by the PackageParser

docs/html/guide/topics/manifest/manifest-element.jd

@@ -44,8 +44,11 @@ to "{@code http://schemas.android.com/apk/res/android}".</dd>
 
 <dt><a name="package"></a>{@code package}</dt>
 <dd>A full Java package name for the application.  The name should 
-be unique.  For example, applications published by Google could have 
-names in the form <code>com.google.app.<i>application_name</i></code>.
+be unique.  The name may contain uppercase or lowercase letters ('A'
+through 'Z'), numbers, and underscores ('_').  However, individual
+package name parts may only start with letters.  For example, applications
+published by Google could have names in the form
+<code>com.google.app.<i>application_name</i></code>.
 
 <p>
 The package name serves as a unique identifier for the application.