Document that PC Card will likely be removed before 13.

Document that PC Card will likely be removed before 13.

This was discussed in arch@ a while ago. Most of the 16-bit drivers that it
relied on have been removed. There's only a few other drivers remaining that
support it, and those are very rare the days (even the once ubiquitious wi(1)
is now quite rare).

Indvidual drivers will be handled separately before pccard itself is removed.