External <use> references -- adding clarifying text to spec

Regarding SVG-ACTION-3794: "Make external <use> be explicitly undefined and
remove crossorigin attribute"

As I discussed with the working group today, I don't want SVG 2 to give the
impression of moving backwards.  We don't want to break things that
currently work because there are undefined edge cases.  So it is important
to be explicit about what is and isn't undefined about external <use>
references.

Therefore, I suggest the following text for an informative note:

The element referenced by `use` may be in a separate document.  However,
this specification does not define how or if the user agent will process
stylesheets and scripts in that document, or what viewport it will use for
resolving percentages and media queries.  When re-using external assets,
therefore, authors are advised not to include any scripts in the external
file and to use inline styles or presentation attributes only.  Percentage
lengths should only be used if the re-used assets define their own viewport
(i.e., if the re-used element is an `svg` or `symbol`).

User agents may restrict restrict external asset files for security
reasons.  Authors are advised that current implementations do not support
cross-origin file access.  A future version of this or another
specification may provide a method of securely enabling cross-origin re-use
of assets.


Again, this is not adding any new limitations, it is just re-iterating and
clarifying the current status quo.  Future versions of the spec may define
the currently undefined behavior.

~ABR