doc: clarify wording about phantom reads

Reported-by: akhilhello@gmail.com

Discussion: https://postgr.es/m/165222922369.669.10475917322916060899@wrigleys.postgresql.org

Backpatch-through: 10

doc/src/sgml/high-availability.sgml

@@ -2345,7 +2345,7 @@ LOG:  database system is ready to accept read only connections
     Currently, temporary table creation is not allowed during read only
     transactions, so in some cases existing scripts will not run correctly.
     This restriction might be relaxed in a later release. This is
-    both a SQL Standard compliance issue and a technical issue.
+    both a SQL standard compliance issue and a technical issue.
    </para>
 
    <para>

doc/src/sgml/mvcc.sgml

@@ -277,9 +277,10 @@
 
    <para>
     The table also shows that PostgreSQL's Repeatable Read implementation
-    does not allow phantom reads.  Stricter behavior is permitted by the
-    SQL standard: the four isolation levels only define which phenomena
-    must not happen, not which phenomena <emphasis>must</emphasis> happen.
+    does not allow phantom reads.  This is acceptable under the SQL
+    standard because the standard specifies which anomalies must
+    <emphasis>not</emphasis> occur at certain isolation levels;  higher
+    guarantees are acceptable.
     The behavior of the available isolation levels is detailed in the
     following subsections.
    </para>