Doc: improve documentation about nextval()/setval().

Clarify that the results of nextval and setval are not guaranteed
persistent until the calling transaction commits.  Some people
seem to have drawn the opposite conclusion from the statement that
these functions are never rolled back, so re-word to avoid saying
it quite that way.

Discussion: https://postgr.es/m/CAKU4AWohO=NfM-4KiZWvdc+z3c1C9FrUBR6xnReFJ6sfy0i=Lw@mail.gmail.com

doc/src/sgml/func.sgml

@@ -17300,24 +17300,37 @@ SELECT setval('myseq', 42, false);    <lineannotation>Next <function>nextval</fu
   <caution>
    <para>
     To avoid blocking concurrent transactions that obtain numbers from
-    the same sequence, a <function>nextval</function> operation is never
-    rolled back; that is, once a value has been fetched it is considered
-    used and will not be returned again.  This is true even if the
-    surrounding transaction later aborts, or if the calling query ends
-    up not using the value.  For example an <command>INSERT</command> with
+    the same sequence, the value obtained by <function>nextval</function>
+    is not reclaimed for re-use if the calling transaction later aborts.
+    This means that transaction aborts or database crashes can result in
+    gaps in the sequence of assigned values.  That can happen without a
+    transaction abort, too.  For example an <command>INSERT</command> with
     an <literal>ON CONFLICT</literal> clause will compute the to-be-inserted
     tuple, including doing any required <function>nextval</function>
     calls, before detecting any conflict that would cause it to follow
-    the <literal>ON CONFLICT</literal> rule instead.  Such cases will leave
-    unused <quote>holes</quote> in the sequence of assigned values.
+    the <literal>ON CONFLICT</literal> rule instead.
     Thus, <productname>PostgreSQL</productname> sequence
     objects <emphasis>cannot be used to obtain <quote>gapless</quote>
     sequences</emphasis>.
    </para>
 
    <para>
-    Likewise, any sequence state changes made by <function>setval</function>
-    are not undone if the transaction rolls back.
+    Likewise, sequence state changes made by <function>setval</function>
+    are immediately visible to other transactions, and are not undone if
+    the calling transaction rolls back.
+   </para>
+
+   <para>
+    If the database cluster crashes before committing a transaction
+    containing a <function>nextval</function>
+    or <function>setval</function> call, the sequence state change might
+    not have made its way to persistent storage, so that it is uncertain
+    whether the sequence will have its original or updated state after the
+    cluster restarts.  This is harmless for usage of the sequence within
+    the database, since other effects of uncommitted transactions will not
+    be visible either.  However, if you wish to use a sequence value for
+    persistent outside-the-database purposes, make sure that the
+    <function>nextval</function> call has been committed before doing so.
    </para>
   </caution>