Add better doc comments/warnings for TransactionWithMeta functions (anza-xyz#4827)

Author: apfitzge

runtime-transaction/src/transaction_with_meta.rs

@@ -6,7 +6,12 @@ use {
 };
 
 pub trait TransactionWithMeta: StaticMeta + SVMTransaction {
-    // Required to interact with geyser plugins.
+    /// Required to interact with geyser plugins.
+    /// This function should not be used except for interacting with geyser.
+    /// It may do numerous allocations that negatively impact performance.
     fn as_sanitized_transaction(&self) -> Cow<SanitizedTransaction>;
+    /// Required to interact with several legacy interfaces that require
+    /// `VersionedTransaction`. This should not be used unless necessary, as it
+    /// performs numerous allocations that negatively impact performance.
     fn to_versioned_transaction(&self) -> VersionedTransaction;
 }