message: Document fixed attribute behavior in critical test framework classes.
  Per suggestion in bitcoin/bitcoin#14305. Also corrects
  module for network objects and wrappers.
diff: @@ -60,6 +60,11 @@ don't have test cases for.
  - When calling RPCs with lots of arguments, consider using named keyword
  arguments instead of positional arguments to make the intent of the call
  clear to readers.
  +- Many of the core test framework classes such as `CBlock` and `CTransaction`
  + don't allow new attributes to be added to their objects at runtime like
  + typical Python objects allow. This helps prevent unpredictable side effects
  + from typographical errors or usage of the objects outside of their intended
  + purpose.
  #### RPC and P2P definitions
  @@ -72,7 +77,7 @@ P2P messages. These can be found in the following source files:
  #### Using the P2P interface
  -- `mininode.py` contains all the definitions for objects that pass
  +- `messages.py` contains all the definitions for objects that pass
  over the network (`CBlock`, `CTransaction`, etc, along with the network-level
  wrappers for them, `msg_block`, `msg_tx`, etc).
  