Table: Describe guarantee about pk and ck column names

Now that we verify there are no gaps in pk and ck, we can explicitly
provide this guarantee to users.

Author: Lorak-mmk

scylla/src/cluster/metadata.rs

@@ -197,7 +197,11 @@ pub struct Keyspace {
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct Table {
     pub columns: HashMap<String, Column>,
+    /// Names of the column of partition key. 
+    /// All of the names are guaranteed to be present in `columns` field.
     pub partition_key: Vec<String>,
+    /// Names of the column of clustering key. 
+    /// All of the names are guaranteed to be present in `columns` field.
     pub clustering_key: Vec<String>,
     pub partitioner: Option<String>,
 }