[PATCH] Attempt to clarify example of serialization anomaly

(sorry to the CCs for sending this twice)

This example has been pointed out as confusing in multiple places, e.g.:

 * https://postgrespro.com/list/thread-id/1845118
 * https://dba.stackexchange.com/a/43951
 * https://stackoverflow.com/a/50733640 ("this paragraph is dark")

As mentioned in those links, it seems to attempt to summarize the
wiki's example at https://wiki.postgresql.org/wiki/SSI#Deposit_Report
. I'm not sure if/how to link to the wiki from the docs, so here is my
humble attempt to summarize it concisely and (hopefully) clearly.

Alternatively, this example could be removed in favor of a reference
to the Serializable section and its example.