manual: document write-only pages

Signed-off-by: Ivan Velickovic <i.velickovic@unsw.edu.au>

manual/parts/vspace.tex

@@ -169,7 +169,8 @@ \subsection{Page}
 are specified with an argument of type \texttt{seL4\_CapRights} given to the mapping invocation. If
 the capability does not have sufficient permissions to authorise the given mapping, the mapping
 permissions are silently downgraded. Specific mapping permissions are dependent on the architecture
-and are documented in the \autoref{sec:api_reference} for each function.
+and are documented in the \autoref{sec:api_reference} for each function. On all architectures,
+mapping a page write-only will result in an inaccessible page.
 
 At minimum, each architecture defines \texttt{Map}, \texttt{Unmap} and
 \texttt{GetAddress} methods for pages.