[misskey] update (#3717)

- add module docstring
- add options to docs/gallery-dl.conf

docs/gallery-dl.conf

@@ -186,6 +186,10 @@
             "username": null,
             "password": null
         },
+        "misskey": {
+            "renotes": false,
+            "replies": true
+        },
         "newgrounds":
         {
             "username": null,

gallery_dl/extractor/misskey.py

@@ -4,6 +4,8 @@
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
+"""Extractors for Misskey instances"""
+
 from .common import BaseExtractor, Message
 from .. import text