[iOS] Explain cert monitoring for accounts in the iOS app

[michaelstingl]

WHAT Needs to be Documented?

iOS app warns users users about significant changes in SSL certificates.
(see information in internal discussion: https://github.com/owncloud/enterprise/issues/4558#issuecomment-839661932)

WHERE Does This Need To Be Documented (Link)?

https://doc.owncloud.com/ios-app/ios_security.html#ssltls-certificates

WHY Should This Change Be Made?

Current docs only cover cert check for new accounts, not changes

(Optional) What Type Of Content Change Is This?

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

(Optional) Which Manual Does This Relate To?

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other