Add login required message when getting comments (#2237)

The comment function in the command line options documentation and the main program file has been edited to state that it now requires login. This change was crucial as the additional requests to Instagram server made for each post necessitate user authentication.

docs/cli-options.rst

@@ -67,7 +67,7 @@ What to Download of each Post
 
    Download and update comments for each post. This requires an additional
    request to the Instagram server for each post, which is why it is disabled by
-   default.
+   default. Requires :option:`--login`.
 
 .. option:: --no-captions
 

instaloader/__main__.py

@@ -367,7 +367,7 @@ def main():
     g_post.add_argument('-C', '--comments', action='store_true',
                         help='Download and update comments for each post. '
                              'This requires an additional request to the Instagram '
-                             'server for each post, which is why it is disabled by default.')
+                             'server for each post, which is why it is disabled by default. Requires --login.')
     g_post.add_argument('--no-captions', action='store_true',
                         help='Do not create txt files.')
     g_post.add_argument('--post-metadata-txt', action='append',

instaloader/structures.py

@@ -685,6 +685,9 @@ class Post:
         .. versionchanged:: 4.7
            Change return type to ``Iterable``.
         """
+        if not self._context.is_logged_in:
+            raise LoginRequiredException("--login required to access comments of a post.")
+
         def _postcommentanswer(node):
             return PostCommentAnswer(id=int(node['id']),
                                      created_at_utc=datetime.utcfromtimestamp(node['created_at']),