[css-values] [editorial] Consider adding non-normative text reminding people that base URLs are mutable

[hober]

§4.4.1 Relative URLs states that "style sheets embedded within a document have the base URL associated with their container." Readers of this specification may be unaware that an HTML container's base URL is mutable (by use of the <base> element). It may be useful to have a non-normative "there be dragons here" note to that effect.