docs: Document unsupported S3 object keys with double slashes

Fixes #5063 by documenting that S3 object keys containing
consecutive forward slashes (//) are not supported by rclone.

The issue occurs because rclone normalizes paths like "a//b" to "a/b",
causing "object not found" errors when trying to access the original
object. This documentation addition explicitly warns users about this
limitation and provides workarounds.

Changes:
- Added new subsection "Important note about double slashes (//)"
  under "Restricted filename characters" in S3 documentation
- Explains the normalization behavior and its consequences
- Provides clear examples and workarounds

AI Model/Tool Attribution:
- Implemented using opencode AI assistant
- Issue analysis and documentation update performed by AI tools

Resolves: #5063

docs/content/s3.md

@@ -671,6 +671,20 @@ work with the SDK properly:
 | .         | ．          |
 | ..        | ．．         |
 
+#### Important note about double slashes (`//`)
+
+Object keys containing consecutive forward slashes (`//`) are **not supported** by rclone. 
+
+When rclone encounters an object key with `//` (e.g., `a//b`), it will normalize the path to use a single slash (e.g., `a/b`). This normalization can cause "object not found" errors when trying to access the original object, as rclone will look for the normalized path instead of the actual object key.
+
+**Example:**
+- Original S3 object key: `folder//file.txt`
+- rclone interprets it as: `folder/file.txt`
+- Result: "object not found" error when trying to access `folder//file.txt`
+
+**Workaround:**
+Avoid using consecutive forward slashes (`//`) in S3 object keys when using rclone. If you have existing objects with `//` in their keys, you will need to rename them to use single slashes or access them through other means.
+
 ### Multipart uploads
 
 rclone supports multipart uploads with S3 which means that it can