[docs] Document function export/entrypoint keyword

Author: PaulBone

docs/plasma_ref.txt

@@ -662,9 +662,13 @@ resource and is already defined, along with it's child resource such as
 === Functions
 
 ----
-FuncDefinition := 'export'? 'func' ident '(' ( Param ( ',' Param )* )? ')'
+FuncDefinition := FuncExport 'func' ident '(' ( Param ( ',' Param )* )? ')'
                       Uses* RetTypes? Block
 
+FuncExport := 'export' 'entrypoint'?
+            | 'entrypoint' 'export'?
+            |
+
 Param := ident ':' TypeExpr
        | '_' : TypeExpr
        | TypeExpr                (Only in interfaces)