feat: add garbage collection info

Fixes blockstack/docs#1123

src/pages/understand-stacks/technical-specs.md

@@ -64,6 +64,7 @@ description: Summary of technical specifications of Stacks 2.0
 - Two types of authorizations: standard authorization is where originating account is the same as paying account. _Sponsored_ authorization is where originating account and paying account are distinct. For instance, developers or service providers could pay for users to call their smart-contracts.
 - For sponsored authorization, first a user signs with the originating account and then a sponsor signs with the paying account.
 - Mempool limit for concurrent pending transactions is 25 per account
+- Pending mempool transactions will be garbage-collected [256 blocks after receipt](https://github.com/blockstack/stacks-blockchain/blob/master/src/core/mempool.rs#L62). With 10 minutes target block time, this would equal ~42 hours
 - Transaction encoding is described [here](https://github.com/stacksgov/sips/blob/main/sips/sip-005/sip-005-blocks-and-transactions.md#transaction-encoding) and [here](/understand-stacks/transactions#encoding)
 - Transaction signing and verification are described [here](https://github.com/stacksgov/sips/blob/main/sips/sip-005/sip-005-blocks-and-transactions.md#transaction-signing-and-verifying) and [here](/understand-stacks/transactions#signature-and-verification)
 - All transactions impacting account balance are atomic, a transfer operation can not increment one account’s balance without decrementing another’s. However, transactions that perform multiple account actions (for example, transferring from multiple accounts) may partially complete.

src/pages/understand-stacks/transactions.md

@@ -535,3 +535,11 @@ Sample response:
   ]
 }
 ```
+
+## Garbage Collection
+
+Broadcasted transactions will stay in the mempool for 256 blocks (~42 hours). If a transactions is not confirmed within that time, it will be removed from the mempool.
+
+!> Most transactions stay in the mempool due to nonce issues. If you see a transaction pending for an unusual time, review the nonce of the account and the transaction.
+
+If a transaction is removed from the mempool, the transaction was not processed and no changes were made to the blockchain state.