add a comment explaining search_path's role in the bug

src/test/regress/expected/drop_if_exists.out

@@ -97,6 +97,8 @@ create database drop_table_test;
 --get the username and set it to a variable
 \set cur_user `echo $USER`
 CREATE SCHEMA :cur_user;
+--the following table will be created in the ":cur_user" schema because
+-- the search_path is '"$user",public'
 CREATE TABLE tbl_to_drop(i int);
 NOTICE:  Table doesn't have 'DISTRIBUTED BY' clause -- Using column named 'i' as the Greenplum Database data distribution key for this table.
 HINT:  The 'DISTRIBUTED BY' clause determines the distribution of data. Make sure column(s) chosen are the optimal data distribution key to minimize skew.

src/test/regress/sql/drop_if_exists.sql

@@ -127,6 +127,8 @@ create database drop_table_test;
 CREATE SCHEMA :cur_user;
 
 
+--the following table will be created in the ":cur_user" schema because
+-- the search_path is '"$user",public'
 CREATE TABLE tbl_to_drop(i int);