Taken from the PR PR 235946, a comment in column.c says:
+/*
+ * Note that c_columnate() and r_columnate() are misleadingly named: the former
+ * fills *rows* first and the latter fills *columns* first. The original names
+ * are kept here only for historical reasons.
+ */
The patch fixes this.
Note: I'm not sure whether we need the CAVEATS section addition, but maybe because of the historic reasons mentioned above, it would make sense.