[Uid] Add note about ULIDs not being random in one case

Author: roverwolf

components/uid.rst

@@ -209,6 +209,13 @@ ULIDs are an alternative to UUIDs when using those is impractical. They provide
 128-bit compatibility with UUID, they are lexicographically sortable and they
 are encoded as 26-character strings (vs 36-character UUIDs).
 
+.. note::
+
+    If you generate more than one ULID during the same millisecond in the
+    same process then the random portion is incremented by one bit in order
+    to provide monotonicity for sorting.  The random portion is **not**
+    random compared to the previous ULID in this case.
+
 Generating ULIDs
 ~~~~~~~~~~~~~~~~