Give VACUUM its own GUC parameter for memory usage, rather than

piggybacking on SortMem.  Add documentation for some recently-added
GUC parameters that had so far escaped it.

src/backend/utils/misc/postgresql.conf.sample

@@ -53,7 +53,8 @@
 #
 #	Performance
 #
-#sort_mem = 512
+#sort_mem = 512             # min 32
+#vacuum_mem = 8192          # min 1024
 #fsync = true
 
 
@@ -173,10 +174,11 @@
 #
 #	Misc
 #
+#dynamic_library_path = '$libdir'
 #australian_timezones = false
 #deadlock_timeout = 1000
 #default_transaction_isolation = 'read committed'
 #max_expr_depth = 10000 # min 10
 #password_encryption = false
 #sql_inheritance = true
-
+#transform_null_equals = false