Manual:$wgExtraSignatureNamespaces

Details
The "Signature" button on the edit toolbar is by default only shown in talk namespaces. It is hidden by default in non-talk namespaces. controls in which subject (non-talk) namespaces the "Signature" button on the edit toolbar will be displayed.

is an array of namespaces, in addition to the talk namespaces, where signatures (~) are likely to be used. This determines whether to display the Signature button on the edit toolbar, and may also be used by extensions. For example, "traditional" style wikis, where content and discussion are intermixed, could place NS_MAIN and NS_PROJECT namespaces in this array.

Example
The configuration will activate the "Signature" button in the Article and the Help namespace.