Clarify add pieces and create set can be made concurrently

its not clear in our docs and code comments that our nonces (and `clientDataSetId`) are not required to be sequential and thus making ppl (and llms) believe these transactions need to be made sequentially.

llms find this bit particularly confusing

https://github.com/FilOzone/synapse-sdk/blob/783ac4ebc27760ecf8e63adefe25be1ad7a4dd75/packages/synapse-sdk/src/storage/context.ts#L131-L149