Groups page: fix label of the mention column

[tidoust]

In groups page such as the mobile one, the Mention column uses a in %page label. Such a label cannot be easily translated into arbitrary languages. A label such as %feature in %page should rather be used.

Note that this will require a second label for cases where the specification is mentioned in more than one page.

[xfq]

Yeah, %feature in %page should be 在%page中的%feature (or just %page中的%feature) in Chinese.