Add docs for "transaction deadline exceeded" error

Fixes #4552.

Summary of changes:

- Update 'Common Errors' page with the retry error message, and
  recommend the user add retries to their app.

v19.1/common-errors.md

@@ -54,6 +54,7 @@ Messages with the error code `40001` and the string `restart transaction` indica
 The sections below describe different types of transaction retry errors.  Your application's retry logic does not need to distinguish between these types of errors; they are listed here for reference.
 
 - [read within uncertainty interval](#read-within-uncertainty-interval)
+- [transaction deadline exceeded](#transaction-deadline-exceeded)
 
 ### read within uncertainty interval
 
@@ -73,6 +74,12 @@ When errors like this occur, the application has the following options:
 Uncertainty errors are a form of transaction conflict. For more information about transaction conflicts, see [Transaction conflicts](architecture/transaction-layer.html#transaction-conflicts).
 {{site.data.alerts.end}}
 
+### transaction deadline exceeded
+
+<span class="version-tag">New in v19.1</span>: Errors of type "transaction deadline exceeded", which were reported to the client as an opaque `TransactionStatusError` in previous versions of CockroachDB, are now considered retryable, and paired with the error code `4001`.  Such errors can occur during long-running transactions (on the order of minutes) when transactions are pushed in the transaction queue, which is necessary to maintain [serializable isolation](https://en.wikipedia.org/wiki/Isolation_(database_systems)#Serializable).
+
+When this error occurs, the application must retry the transaction. For more information about how to retry transactions, see [Transaction retries](transactions.html#transaction-retries).
+
 <!-- ### write too old -->
 
 <!-- ### async write failure -->