Revert "Documentation/features/vm: Remove arch support status file for 'pte_special'"

The removal of this file appears to have been premature; it's not a feature
enabled by Kconfig, but it's a arch-level feature regardless.  Put it back
for now until some happy future time when we decide how we really want to
document such features.

This reverts commit 2bef69a3.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>