Note the limitation that you can't exclude multipart support

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/docs/proposed/psa-conditional-inclusion-c.md b/docs/proposed/psa-conditional-inclusion-c.md
index 61d3d17..7ba52d0 100644
--- a/docs/proposed/psa-conditional-inclusion-c.md
+++ b/docs/proposed/psa-conditional-inclusion-c.md
@@ -41,6 +41,8 @@
 
 [Limitation.size] There is no requirement to include only support for certain key sizes.
 
+[Limitation.multipart] Where there are multiple ways to perform an operation, for example single-part and multi-part, there is no mechanism to select only one or a subset of the possible ways.
+
 ## Interface
 
 ### PSA Crypto configuration file