Add docs

src/db.rs

@@ -836,6 +836,7 @@ impl DB {
         &self.path.as_path()
     }
 
+    /// Flush database memtable to SST files on disk (with options).
     pub fn flush_opt(&self, flushopts: &FlushOptions) -> Result<(), Error> {
         unsafe {
             ffi_try!(ffi::rocksdb_flush(self.inner, flushopts.inner,));
@@ -843,6 +844,7 @@ impl DB {
         Ok(())
     }
 
+    /// Flush database memtable to SST files on disk.
     pub fn flush(&self) -> Result<(), Error> {
         self.flush_opt(&FlushOptions::default())
     }