Add note about ULIDs not being random in one case

Without reading the ULID specification, it is not obvious in this documentation that the random portion of a ULID is not entirely random when multiple are generated in the same millisecond.

Author: roverwolf

components/uid.rst

@@ -209,6 +209,12 @@ 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 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
 ~~~~~~~~~~~~~~~~