Template:Decade by city category header/doc

Creates a category of the form "YYY0s by city", where YYY0s is a 4-digit decade.

e.g.
 * Category:1810s by city
 * Category:1990s by city

Usage
No parameters needed

Errors
If this template is used on an invalid page, then an error message is displayed, and the page is categorised in a tracking category: Note that this tracking page is shared with other category header templates)
 * Category:Errors reported by other category header templates (population: pages)

Errors include:
 * The page is not in the category namespace
 * The page's title does not take the form "YYY0s by city"