correct the directory mount point rule

Mount-on-Windows.md

@@ -21,10 +21,10 @@ weed.exe mount -filer=127.0.0.1:8888 -dir=S:
 The mount point is one of:
 
 * a **drive letter**, `S:`, which must be free
-* a **directory that does not exist yet**, `C:\seaweed\data` — its parent must exist, and WinFsp creates it
+* a **directory**, `C:\seaweed\data` — either an empty one, or one that does not exist yet and whose parent does
 * a **UNC path**, `\\seaweedfs\data`
 
-An existing directory is rejected: WinFsp cannot mount over one.
+A directory mount point becomes a reparse point for as long as the mount lasts, and NTFS only allows that on an empty directory, so a populated one is rejected. WinFsp creates a missing directory and removes it again when the filesystem goes away.
 
 Prefer `127.0.0.1` over `localhost`. Windows resolves `localhost` to `::1` first, and a filer listening only on IPv4 will refuse the connection.