chain-spec: minor clarification on the genesis config patch (#5324)

Added minor clarification on the genesis config patch
([link](https://substrate.stackexchange.com/questions/11813/in-the-genesis-config-what-does-the-patch-key-do/11825#11825))

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Co-authored-by: command-bot <>

substrate/client/chain-spec/src/lib.rs

@@ -172,6 +172,12 @@
 //!   </tbody>
 //! </table>
 //!
+//! The main purpose of the `RuntimeGenesisConfig` patch is to:
+//! - minimize the maintenance effort when RuntimeGenesisConfig is changed in the future (e.g. new
+//!   pallets added to the runtime or pallet's genesis config changed),
+//! - increase the readability - it only contains the relevant fields,
+//! - allow to apply numerous changes in distinct domains (e.g. for zombienet).
+//!
 //! For production or long-lasting blockchains, using the `raw` format in the chain specification is
 //! recommended. Only the `raw` format guarantees that storage root hash will remain unchanged when
 //! the `RuntimeGenesisConfig` format changes due to software upgrade.