Improve some python documentation and add an intro

TODO.md

@@ -15,6 +15,8 @@
 
 ## Issues
 
+- SSL error on python 3.7
+
 - Jumping between accounts (clicking in account bar or alt+(Shift+)N) is
   laggy with hundreds of rooms in between
 - On startup, if a room's last event is a membership change,

src/backend/__init__.py

@@ -1,6 +1,15 @@
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
-"""This package provides a Python backend accessible from the QML UI side."""
+"""This package provides Mirage's backend side that can interact with the UI.
+
+To learn more about how this package works, you might want to check the
+documentation in the following modules first:
+
+- `qml_bridge`
+- `backend`
+- `matrix_client`
+- `nio_callbacks`
+"""
 
 __app_name__     = "mirage"
 __display_name__ = "Mirage"

src/backend/matrix_client.py

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: LGPL-3.0-or-later
 
-"""Matrix client and related classes."""
+"""Matrix client to interact with a homeserver and other related classes."""
 
 import asyncio
 import html

src/backend/nio_callbacks.py

@@ -22,7 +22,7 @@ if TYPE_CHECKING:
 class NioCallbacks:
     """Register callbacks for nio's request responses and room events.
 
-    For every nio `Response` and `Event` subclasses, this class can have a
+    For every `nio.Response` and `nio.Event` subclasses, this class can have a
     method named `on<ClassName>` (e.g. `onRoomMessageText`) that will
     automatically be registered in nio.
 

src/backend/qml_bridge.py

@@ -4,6 +4,15 @@
 # directly or indirectly via another module import (e.g. backend).
 # See https://stackoverflow.com/a/55918049
 
+"""Provide `BRIDGE`, main object accessed by QML to interact with Python.
+
+PyOtherSide, the library that handles interaction between our Python backend
+and QML UI, will access the `BRIDGE` object and call its methods directly.
+
+The `BRIDGE` object should be the only existing instance of the `QMLBridge`
+class.
+"""
+
 import asyncio
 import logging as log
 import os
@@ -18,16 +27,16 @@ from .pyotherside_events import CoroutineDone, LoopException
 
 
 class QMLBridge:
-    """Setup asyncio and provide synchronous methods to call coroutines.
+    """Setup asyncio and provide methods to call coroutines from QML.
 
     A thread is created to run the asyncio loop in, to ensure all calls from
     QML return instantly.
-    Methods are provided for QML to call coroutines using PyOtherSide, which
-    doesn't have this ability out of the box.
+    Synchronous methods are provided for QML to call coroutines using
+    PyOtherSide, which doesn't have this ability out of the box.
 
     Attributes:
-        backend: The `Backend` containing the coroutines of interest and
-            `MatrixClient` objects.
+        backend: The `backend.Backend` object containing general coroutines
+            for QML and that manages `MatrixClient` objects.
     """
 
     def __init__(self) -> None: