fix: Correct the type annotations for extension parameters

[silasary]

What type of pull request is this?

• Non-breaking code change
• Breaking code change
• Documentation change/addition
• Tests change

Description

When annotating a **param, you don't need to state that it's a mapping of string, that's implicit.
The annotation is automatically Mapping[str, T]. So we were accidentally saying that people could only have dicts as parameters to load_extension.

Also some docs improvements while I'm in here :P

Changes

• docs: Remove duplicated parameters and add send_command_tracebacks to Client.init's documentation
• fix: Correct the type annotations for extension parameters

Checklist

• I've formatted my code with Black
• I've ensured my code works on Python 3.10.x
• I've tested my code