Chore: Cleanup command arguments and standardize process count handling (#4541)

Cleans up some command help text and adds more control over process count for command with a Pool

src/paperless_mail/management/commands/mail_fetcher.py

@@ -4,11 +4,7 @@ from paperless_mail import tasks
 
 
 class Command(BaseCommand):
-    help = """
-    """.replace(
-        "    ",
-        "",
-    )
+    help = "Manually triggers a fetching and processing of all mail accounts"
 
     def handle(self, *args, **options):
         tasks.process_mail_accounts()