Tweak description comments in tests for partition functions

The new wording is more generic and fixes one grammar mistake and one
typo on the way.

Per discussion between Amit Langote and me.

Discussion: https://postgr.es/m/20181217064028.GJ31474@paquier.xyz

src/test/regress/expected/partition_info.out

 --
--- Tests for pg_partition_tree
+-- Tests for functions providing information about partitions
 --
 SELECT * FROM pg_partition_tree(NULL);
  relid | parentrelid | isleaf | level 
@@ -99,7 +99,7 @@ SELECT relid, parentrelid, level, isleaf
 (1 row)
 
 DROP TABLE ptif_test;
--- A table not part of a partition tree works is the only member listed.
+-- Table that is not part of any partition tree is the only member listed.
 CREATE TABLE ptif_normal_table(a int);
 SELECT relid, parentrelid, level, isleaf
   FROM pg_partition_tree('ptif_normal_table');
@@ -109,7 +109,9 @@ SELECT relid, parentrelid, level, isleaf
 (1 row)
 
 DROP TABLE ptif_normal_table;
--- Views and materialized viewS cannot be part of a partition tree.
+-- Various partitioning-related functions return NULL if passed relations
+-- of types that cannot be part of a partition tree; for example, views,
+-- materialized views, etc.
 CREATE VIEW ptif_test_view AS SELECT 1;
 CREATE MATERIALIZED VIEW ptif_test_matview AS SELECT 1;
 SELECT * FROM pg_partition_tree('ptif_test_view');

src/test/regress/sql/partition_info.sql

 --
--- Tests for pg_partition_tree
+-- Tests for functions providing information about partitions
 --
 SELECT * FROM pg_partition_tree(NULL);
 SELECT * FROM pg_partition_tree(0);
@@ -54,13 +54,15 @@ SELECT relid, parentrelid, level, isleaf
 
 DROP TABLE ptif_test;
 
--- A table not part of a partition tree works is the only member listed.
+-- Table that is not part of any partition tree is the only member listed.
 CREATE TABLE ptif_normal_table(a int);
 SELECT relid, parentrelid, level, isleaf
   FROM pg_partition_tree('ptif_normal_table');
 DROP TABLE ptif_normal_table;
 
--- Views and materialized viewS cannot be part of a partition tree.
+-- Various partitioning-related functions return NULL if passed relations
+-- of types that cannot be part of a partition tree; for example, views,
+-- materialized views, etc.
 CREATE VIEW ptif_test_view AS SELECT 1;
 CREATE MATERIALIZED VIEW ptif_test_matview AS SELECT 1;
 SELECT * FROM pg_partition_tree('ptif_test_view');