docs(backup): updated docs with a disclaimer about version compatibility

.docs/backup.md

@@ -10,6 +10,14 @@ Backup allows you to export a JSON list of all of your installed repositories an
 [p]cog load backup
 ```
 
+## Version Compatibility
+
+As of commit [1edb08a](https://www.coastalcommits.com/SeaswimmerTheFsh/SeaCogs/commit/1edb08a1271f12098ca0bed11a735f7162cedd14), the Backup cog no longer supports Red versions older than 3.5.6. If you want to use the cog on an earlier version (3.5.0 - 3.5.5), install the cog pinned to this commit: `43464db6a7c51bc69282b1ae3dc507a4aae851de`.
+
+```bash
+[p]cog installversion sea-cogs 43464db6a7c51bc69282b1ae3dc507a4aae851de backup
+```
+
 ## Commands
 
 ### backup export

backup/backup.py

@@ -14,8 +14,7 @@ from redbot.cogs.downloader import errors
 from redbot.cogs.downloader.converters import InstalledCog
 from redbot.core import commands
 from redbot.core.bot import Red
-from redbot.core.utils.chat_formatting import (error, humanize_list,
-                                               text_to_file)
+from redbot.core.utils.chat_formatting import error, humanize_list, text_to_file
 
 
 # pylint: disable=protected-access
@@ -159,17 +158,15 @@ class Backup(commands.Cog):
                     )
                     self.logger.debug("Repository %s already exists", name)
 
-                # This is commented out because errors.AuthenticationError is not yet implemented in Red 3.5.5's Downloader cog.
-                # Rather, it is only in the development version and will be added in version 3.5.6 (or whatever the next version is).
-                # except errors.AuthenticationError as err:
-                #     repo_e.append(f"Authentication error while adding repository {name}. See logs for more information.")
-                #     self.logger.exception(
-                #         "Something went wrong whilst cloning %s (to revision %s)",
-                #         url,
-                #         branch,
-                #         exc_info=err,
-                #     )
-                #     continue
+                except errors.AuthenticationError as err:
+                    repo_e.append(f"Authentication error while adding repository {name}. See logs for more information.")
+                    self.logger.exception(
+                        "Something went wrong whilst cloning %s (to revision %s)",
+                        url,
+                        branch,
+                        exc_info=err,
+                    )
+                    continue
 
                 except errors.CloningError as err:
                     repo_e.append(