docs: add for SendSubmissionParamsInput::with_message method

During the discussion, it turned out that we did not have a high-level description for the concept. I left a simple one, however, as soon as the description appears in the gitbook we have added a link here

Close #24

src/sending.rs

@@ -13,9 +13,9 @@ use crate::{
         INIT_EXTERNAL_CALL_DISCRIMINATOR, SEND_DISCRIMINATOR,
     },
     errors::InvokeError,
+    flags::SetReservedFlag,
     hash::HashAdapter,
     keys::{AssetFeeInfoPubkey, BridgePubkey, ChainSupportInfoPubkey},
-    flags::SetReservedFlag,
     Error, Pubkey, SolanaKeccak256, BPS_DENOMINATOR, DEBRIDGE_ID, SOLANA_CHAIN_ID,
 };
 
@@ -86,6 +86,12 @@ impl SendSubmissionParamsInput {
 
     /// Create submission params for sending message to other chain
     ///
+    /// This path is the default path for sending an authorised message
+    /// on the EVM network. If your case is simple, this is probably the
+    /// method you need.
+    ///
+    /// For everything else, please use [`SendSubmissionParamsInput::with_external_call`]
+    ///
     /// # Arguments
     /// * `external_call` - instructions sending in target chain
     /// * `execution_fee` - amount of execution fee