fix: HIP-540: clarify that an admin key cannot be set to "unusable" k…

…ey value (#969)

Signed-off-by: Petar Tonev <[email protected]>

HIP/hip-540.md

@@ -11,7 +11,7 @@ status: Accepted
 last-call-date-time: 2023-07-03T07:00:00Z
 created: 2022-08-05
 discussions-to: https://github.com/hashgraph/hedera-improvement-proposal/discussions/522
-updated: 2024-04-26
+updated: 2024-05-15
 ---
 
 ## Abstract
@@ -90,7 +90,7 @@ First, let's address important language to set a clear distinction between "remo
 ## Specification
 
 1. Only the admin key should be able to remove itself or other keys.
-2. All keys can change themselves to another valid or unusable key (such as all-zeros).
+2. All keys can change themselves to another valid or unusable key (such as all-zeros). Except for the admin key - we do not support setting an unusable admin key. This is because removing the admin key and making it unusable are functionally identical. It should simply be removed if no longer desired.
 
 ### Other Considerations