Rename bot.db as bot._config (#2967)

* Rename `bot.db` as `bot._config`

  - Continues work towards strong version guarantees
  - Added methods for cog use for a few things which were previously
  only accessible via direct access.
  - Retained private use in a few internal use locations, though most
  methods were updated away from this.
  - Updated documentation for shared api token users

* changelog

* more detail

* docstring fixes

* Apparently, I forgot to commit something I had locally

  - + a copy/paste failue in the changelog

* *sigh*:

* *sigh*

redbot/__main__.py

@@ -52,8 +52,8 @@ async def _get_prefix_and_token(red, indict):
     :param indict:
     :return:
     """
-    indict["token"] = await red.db.token()
-    indict["prefix"] = await red.db.prefix()
+    indict["token"] = await red._config.token()
+    indict["prefix"] = await red._config.prefix()
 
 
 def list_instances():
@@ -115,7 +115,7 @@ def main():
     red = Red(
         cli_flags=cli_flags, description=description, dm_help=None, fetch_offline_members=True
     )
-    loop.run_until_complete(red.maybe_update_config())
+    loop.run_until_complete(red._maybe_update_config())
     init_global_checks(red)
     init_events(red, cli_flags)
 
@@ -153,11 +153,11 @@ def main():
         loop.run_until_complete(red.start(token, bot=True))
     except discord.LoginFailure:
         log.critical("This token doesn't seem to be valid.")
-        db_token = loop.run_until_complete(red.db.token())
+        db_token = loop.run_until_complete(red._config.token())
         if db_token and not cli_flags.no_prompt:
             print("\nDo you want to reset the token? (y/n)")
             if confirm("> "):
-                loop.run_until_complete(red.db.token.set(""))
+                loop.run_until_complete(red._config.token.set(""))
                 print("Token has been reset.")
     except KeyboardInterrupt:
         log.info("Keyboard interrupt detected. Quitting...")