module_utils/ec2: fix boto3 tags to remove (#25277)

* Fix boto3 tags to remove; unable to remove a list of strings, need list of {'Key': key}

* pep8 fix

* cannot bugfix due to differences in AWS APIs - just document

* add more info to guidelines for using compare_aws_tags

lib/ansible/module_utils/ec2.py

@@ -652,13 +652,14 @@ def map_complex_type(complex_type, type_map):
 def compare_aws_tags(current_tags_dict, new_tags_dict, purge_tags=True):
     """
     Compare two dicts of AWS tags. Dicts are expected to of been created using 'boto3_tag_list_to_ansible_dict' helper function.
-    Two dicts are returned - the first is tags to be set, the second is any tags to remove
+    Two dicts are returned - the first is tags to be set, the second is any tags to remove. Since the AWS APIs differ t
+hese may not be able to be used out of the box.
 
     :param current_tags_dict:
     :param new_tags_dict:
     :param purge_tags:
     :return: tag_key_value_pairs_to_set: a dict of key value pairs that need to be set in AWS. If all tags are identical this dict will be empty
-    :return: tag_keys_to_unset: a list of key names that need to be unset in AWS. If no tags need to be unset this list will be empty
+    :return: tag_keys_to_unset: a list of key names (type str) that need to be unset in AWS. If no tags need to be unset this list will be empty
     """
 
     tag_key_value_pairs_to_set = {}

lib/ansible/modules/cloud/amazon/GUIDELINES.md

@@ -314,4 +314,6 @@ to modify and a list of tag key names that you need to remove.  Purge is True by
 existing tags will not be modified.
 
 This function is useful when using boto3 'add_tags' and 'remove_tags' functions. Be sure to use the other helper function
-'boto3_tag_list_to_ansible_dict' to get an appropriate tag dict before calling this function.
+'boto3_tag_list_to_ansible_dict' to get an appropriate tag dict before calling this function. Since the AWS APIs are not
+uniform (e.g. EC2 versus Lambda) this will work without modification for some (Lambda) and others may need modification
+before using these values (such as EC2, with requires the tags to unset to be in the form [{'Key': key1}, {'Key': key2}]).