[css-namespaces-3] Replace CSS 2.1 syntax definition by CSS syntax

[SebastianZ]

The minutes from 2016-10-12 resolved on removing the CSS grammar section from CSS 2.2 and instead refer to the CSS Syntax module. Though the CSS Namespaces Module still uses that grammar for the definition of the @namespace rule.

This syntax should be replaced by something like this:

@namespace <namespace-prefix>? [ <string> | <url> ];

where
<namespace-prefix> = <ident>

Sebastian

[tabatkins]

Ping @fantasai since this came up again, and she's convinced it can be an editorial edit if we don't remove the Appendix G-style grammar, and thus wouldn't be too hard to update the Rec with.