sync: add page (#1507)

pages/common/sync.md

@@ -0,0 +1,11 @@
+# sync
+
+> Flushes all pending write operations to the appropriate disks.
+
+- Flush all pending write operations on all disks:
+
+`sync`
+
+- Flush all pending write operations on a single file to disk:
+
+`sync {{path/to/file}}`