message: Add comments on CPubKey::IsValid
diff: @@ -170,6 +170,15 @@ public:
  /*
  * Check syntactic correctness.
  *
  + * When setting a pubkey (Set()) or deserializing fails (its header bytes
  + * don't match the length of the data), the size is set to 0. Thus,
  + * by checking size, one can observe whether Set() or deserialization has
  + * failed.
  + *
  + * This does not check for more than that. In particular, it does not verify
  + * that the coordinates correspond to a point on the curve (see IsFullyValid()
  + * for that instead).
  + *
  * Note that this is consensus critical as CheckECDSASignature() calls it!
  */
  bool IsValid() const
  