[css3-selectors] escapes not defined

[fantasai]

The Selectors Level 3 spec includes CSS escapes in its grammar, but
there is no mention of them in the prose. I've added the following
paragraph to the Syntax section:

   # Characters in Selectors can be escaped with a backslash according
   # to the same escaping rules as CSS. [CSS21].

where "escaping rules" is linked to http://www.w3.org/TR/CSS21/syndata.html#characters

Let me know if this is a problem.

~fantasai