registration: Fix namespaces added through the ExtensionProcessor

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
by MWNamespace::getCanonicalNamespaces().

Also fix the documentation on $wgExtraNamespaces, as using
$wgCanonicalNamespaceNames has not been possible since r85327.

Bug: T110389
Change-Id: I5bd9a7258f59d8c4a7ad0543d2115960fbea9b3a