Document "B" and "us" as accepted units in postgres.conf.sample

In postgresql.conf, memory and file size GUCs can be specified with "B"
(bytes) as of b06d8e58b. Likewise, time GUCs can be specified with "us"
(microseconds) as of caf626b2c. Update postgres.conf.sample to reflect
that fact.

Pavel Luzanov

Backpatch to v12, which is the earliest version that allows both of
these units. A separate commit will document the "B" case for v11.

Discussion: https://www.postgresql.org/message-id/flat/f10d16fc-8fa0-1b3c-7371-cb3a35a13b7a%40postgrespro.ru

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

@@ -24,7 +24,8 @@
 # "postgres -c log_connections=on".  Some parameters can be changed at run time
 # with the "SET" SQL command.
 #
-# Memory units:  kB = kilobytes        Time units:  ms  = milliseconds
+# Memory units:  B  = bytes            Time units:  us  = microseconds
+#                kB = kilobytes                     ms  = milliseconds
 #                MB = megabytes                     s   = seconds
 #                GB = gigabytes                     min = minutes
 #                TB = terabytes                     h   = hours