registration: Fix namespaces added through the ExtensionProcessor
Using $wgExtraNamespaces overrides any localized namespaces with the
canonical form, which is not ideal.
Namespaces added through extension.json will now store the canonical
form and numerical id in a 'ExtensionNamespaces' attribute that is read
Also fix the documentation on $wgExtraNamespaces, as using
$wgCanonicalNamespaceNames has not been possible since r85327.