feat(filesystem): document write flags precise meaning

Author: MendyBerger

proposals/filesystem/wit/types.wit

@@ -94,6 +94,9 @@ interface types {
         requested-write-sync,
         /// Mutating directories mode: Directory contents may be mutated.
         ///
+        /// Governs directory structure (create, rename, delete entries); independent
+        /// of `write`, which governs file contents.
+        ///
         /// When this flag is unset on a descriptor, operations using the
         /// descriptor which would create, rename, delete, modify the data or
         /// metadata of filesystem objects, or obtain another handle which