Clarify block issuance credit < 0 behavior

tips/TIP-0042/tip-0042.md

@@ -149,11 +149,15 @@ always need to be one _Account Unlock_ referencing a greater index.
 
 ## Block Issuer Feature
 
-The presence of a <i>Block Issuer Feature</i> on an account signals that this account can issue blocks - it is an
-_issuer account_. This feature defines the public keys with which a signature to burn Mana from the containing account's
-Block Issuance Credit can be verified. The feature is locked to the account to disincentivize malicious behavior when
-issuing blocks. The feature can be unlocked with an expiration mechanism. Any Mana generated or stored by an issuer
-account is locked to that account.
+The presence of a _Block Issuer Feature_ on an account signals that this account can issue blocks - it is a _block
+issuer account_. This feature defines the public keys with which signatures on blocks can be verified. A block issued
+from this account with a verified signature will burn Mana from the account's Block Issuance Credit (BIC) balance. If
+the balance becomes negative, the account is locked, meaning its output cannot be spent and it can no longer be used to
+issue blocks. Once locked, an account can only be unlocked by allotting enough Mana to the account's BIC balance for it
+to become non-negative again. Because the account itself cannot issue such a transaction, the block containing the
+alloting transaction must be issued by another account. The feature can only be removed with an expiration mechanism in
+order to disincentivize malicious behavior when issuing blocks. Any Mana generated or stored by an issuer account is
+locked to that account.
 
 <details>
   <summary>Block Issuer Feature</summary>