docs: add FOR UPDATE clause to MySQL transaction example

Fixes issue where transaction example doesn't properly lock rows
in MySQL, which could lead to race conditions when reading and
then updating the same data.

The SELECT statement in the transaction example now includes
FOR UPDATE clause to provide exclusive row locking in MySQL/InnoDB,
as recommended in the MySQL documentation for safe read-then-update
operations within transactions.

Also adds explanation about locking reads in transactions to help
users understand when and why to use SELECT ... FOR UPDATE.

Fixes #3485

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: kotahorii

docs/howto/transactions.md

@@ -1,6 +1,10 @@
 # Using transactions
 In the code generated by sqlc, the `WithTx` method allows a `Queries` instance to be associated with a transaction.
 
+## Locking Reads in Transactions
+
+When you query data and then update related data within the same transaction, regular `SELECT` statements may not provide adequate protection against race conditions. For MySQL/InnoDB, using `SELECT ... FOR UPDATE` provides exclusive locks on the selected rows, preventing other transactions from modifying them until your transaction completes.
+
 For example, with the following SQL structure:
 
 `schema.sql`:
@@ -15,7 +19,8 @@ CREATE TABLE records (
 ```sql
 -- name: GetRecord :one
 SELECT * FROM records
-WHERE id = $1;
+WHERE id = $1
+FOR UPDATE;
 
 -- name: UpdateRecord :exec
 UPDATE records SET counter = $2