Specify the registry.json file encoding (#23984)

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## Description

Added a mention of exactly which file encoding we expect for the user
created `registry.json` file.

## Related issues or tickets

https://docker.atlassian.net/browse/DDB-181
Similar to #23983 

## Reviews

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- [ ] Technical review
- [ ] Editorial review
- [ ] Product review

Author: mat007

content/manuals/enterprise/security/enforce-sign-in/methods.md

@@ -220,7 +220,7 @@ The registry.json method works across all platforms and offers flexible deployme
 
 ### File locations
 
-Create the `registry.json` file at the appropriate location:
+Create the `registry.json` file (UTF-8 without BOM) at the appropriate location:
 
 | Platform | Location |
 | --- | --- |