docs(transactions): note the lost-update risk in read-modify-write

[shaikn6]

What

The Using transactions how-to's bumpCounter example reads a row and then writes counter + 1 inside a transaction:

r, err := qtx.GetRecord(ctx, id)
...
qtx.UpdateRecord(ctx, tutorial.UpdateRecordParams{ID: r.ID, Counter: r.Counter + 1})

Under the default isolation level (READ COMMITTED, on both PostgreSQL and MySQL/InnoDB), two transactions running this concurrently can both read the same counter and both write the same counter + 1 — a lost update. The page presents this as the standard pattern with no caveat, which is what #3485 flags.

Change

Docs only. Adds a short Concurrent read-modify-write section after the examples:

• explains the lost-update window
• shows a GetRecordForUpdate query using SELECT ... FOR UPDATE
• mentions SERIALIZABLE + retry as the alternative, noting sqlc doesn't manage isolation levels or retries

Existing examples are unchanged — they still demonstrate WithTx, which is the page's purpose.

Closes #3485