Update transaction size limit explanation (#942)

Clarified the transaction size limit explanation to include details about IPv6 MTU and network headers.

Author: leafaar

apps/web/content/docs/en/core/transactions.mdx

@@ -55,8 +55,12 @@ This limit includes both the [`signatures`](#signatures) array
 and the [`message`](#message) struct.
 
 <Callout>
-This limit comes from the IPv6 Maximum Transmission Unit (MTU) size of
-1280 bytes, minus 48 bytes for network headers (40 bytes IPv6 + 8 bytes header).
+This limit is designed to avoid packet fragmentation on typical internet infrastructure.
+While IPv6 supports MTUs larger than 9000 bytes, most internet routers use a default
+MTU of 1500 bytes (standard Ethernet). To ensure transactions fit within a single packet
+without fragmentation, Solana uses 1280 bytes (the minimum MTU required for IPv6) minus
+48 bytes for network headers (40 bytes IPv6 + 8 bytes fragment/UDP header), resulting
+in the 1232 byte transaction size limit.
 </Callout>
 
 ![Diagram showing the transaction format and size limits](/assets/docs/core/transactions/issues_with_legacy_txs.png)