add description for WriteResult enum

Author: ronaudinho

src/inmem/mutable.rs

@@ -31,6 +31,7 @@ pub(crate) type MutableScan<'scan, R> = Range<
     Option<R>,
 >;
 
+/// Signals to memtable whether size threshold has been reached and needs compaction
 #[derive(Debug)]
 pub enum WriteResult {
     Continue,