Fixes #67. Also, prevent people from accidentally making MessageBuilder…

Fixes #67.  Also, prevent people from accidentally making MessageBuilder subclasses copyable/movable as brought up in issue #64.

c++/src/capnp/layout.c++

@@ -235,7 +235,6 @@ struct WirePointer {
     uint32_t upper32Bits;
 
     StructRef structRef;
-    // Also covers capabilities.
 
     ListRef listRef;
 

c++/src/capnp/message.h

@@ -157,6 +157,7 @@ class MessageBuilder {
 public:
   MessageBuilder();
   virtual ~MessageBuilder() noexcept(false);
+  KJ_DISALLOW_COPY(MessageBuilder);
 
   virtual kj::ArrayPtr<word> allocateSegment(uint minimumSize) = 0;
   // Allocates an array of at least the given number of words, throwing an exception or crashing if

c++/src/capnp/rpc.capnp

@@ -851,11 +851,11 @@ struct Payload {
 struct CapDescriptor {
   # **(level 1)**
   #
-  # When an application-defined type contains an interface pointer, that pointer's encoding is the
-  # same as a struct pointer except that the bottom two bits are 1's instead of 0's.  The pointer
-  # actually points to an instance of `CapDescriptor`.  The runtime API should not reveal the
-  # CapDescriptor directly to the application, but should instead wrap it in some kind of callable
-  # object with methods corresponding to the interface that the capability implements.
+  # When an application-defined type contains an interface pointer, that pointer contains an index
+  # into the message's capability table -- i.e. the `capTable` part of the `Payload`.  Each
+  # capability in the table is represented as a `CapDescriptor`.  The runtime API should not reveal
+  # the CapDescriptor directly to the application, but should instead wrap it in some kind of
+  # callable object with methods corresponding to the interface that the capability implements.
   #
   # Keep in mind that `ExportIds` in a `CapDescriptor` are subject to reference counting.  See the
   # description of `ExportId`.