Issue p4lang#722: Document restrictions on types IH and EH in PSA

p4-16/psa/PSA.mdk

@@ -615,6 +615,16 @@ for all ingress parsers and control blocks.  The egress parser and
 control blocks can use the same types for those things, or different
 types, as the P4 program author wishes.
 
+The types `IH` and `EH` are restricted to be structs.  For each member
+of these struct types, it must be one of the following types:
+
++ `header`
++ `header_union`
++ header stack
+
+The intent is that these structs contain all of the headers that can
+be parsed, or deparsed.
+
 ```
 [INCLUDE=psa.p4:Programmable_blocks]
 ```