prepared: docs mention about metadata immutability

Added docstring mention about metadata immutability and when not to
enable `cached_result_metadata` optimisation on the statement.

scylla/src/statement/prepared_statement.rs

@@ -283,6 +283,17 @@ impl PreparedStatement {
     /// after statement preparation and will use it
     /// to deserialize the results of statement execution.
     ///
+    /// NOTICE: Users should NOT alter the schema after enablind
+    /// this optimisation on the statement.
+    /// To be more precise:
+    /// they should not alter the tables and UDTs that are used in the given statement.
+    /// The reason for this is that the decision was made to keep client-side metadata immutable,
+    /// and so, it is not updated during statement repreparation.
+    /// Altering the schema, and then, executing the statement with optimisation enabled may lead
+    /// to runtime errors. In the worst case, it could lead to
+    /// some data being deserialized to wrong Rust type contained
+    /// in outdated metadata.
+    ///
     /// This option is false by default.
     pub fn set_use_cached_result_metadata(&mut self, use_cached_metadata: bool) {
         self.config.skip_result_metadata = use_cached_metadata;