C comment: adjust statistics mention

No need to talk about the statistics collector.

Discussion: https://postgr.es/m/8a82417cdb6e8038fe276d4960e3207a@oss.nttdata.com

Author: Álvaro Herrera

Backpatch-through: master

src/include/catalog/pg_statistic.h

@@ -178,7 +178,7 @@ DECLARE_FOREIGN_KEY((starelid, staattnum), pg_attribute, (attrelid, attnum));
  * the K most common non-null values appearing in the column, and stanumbers
  * contains their frequencies (fractions of total row count).  The values
  * shall be ordered in decreasing frequency.  Note that since the arrays are
- * variable-size, K may be chosen by the statistics collector.  Values should
+ * variable-size, K may be chosen may be chosen at ANALYZE time.  Values should
  * not appear in MCV unless they have been observed to occur more than once;
  * a unique column will have no MCV slot.
  */