Mention tx_max_bytes in the tutorials (tendermint#9809)

Updates the discussion of prepareProposal in the go tutorials to mention tx_max_bytes limitation to transaction set size.



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#### PR checklist

- [x] Tests written/updated, or no tests needed
- [x] `CHANGELOG_PENDING.md` updated, or no changelog entry needed
- [x] Updated relevant documentation (`docs/`) and code comments, or no
      documentation updates needed

docs/tutorials/go-built-in.md

@@ -461,7 +461,8 @@ When Tendermint Core sees that valid transactions (validated through `CheckTx`)
 included in blocks, it groups some of these transactions and then gives the application a chance 
 to modify the group by invoking `PrepareProposal`.
 
-The application is free to modify the group before returning from the call.
+The application is free to modify the group before returning from the call, as long as the resulting set
+does not use more bytes than `RequestPrepareProposal.max_tx_bytes'
 For example, the application may reorder, add, or even remove transactions from the group to improve the
 execution of the block once accepted.
 In the following code, the application simply returns the unmodified group of transactions:

docs/tutorials/go.md

@@ -460,7 +460,8 @@ When Tendermint Core sees that valid transactions (validated through `CheckTx`)
 included in blocks, it groups some of these transactions and then gives the application a chance 
 to modify the group by invoking `PrepareProposal`.
 
-The application is free to modify the group before returning from the call.
+The application is free to modify the group before returning from the call, as long as the resulting set
+does not use more bytes than `RequestPrepareProposal.max_tx_bytes'
 For example, the application may reorder, add, or even remove transactions from the group to improve the
 execution of the block once accepted.
 In the following code, the application simply returns the unmodified group of transactions: