Tweak wording in sequence-function docs to avoid PDF build failures.

Adjust the wording in the first para of "Sequence Manipulation Functions"
so that neither of the link phrases in it break across line boundaries,
in either A4- or US-page-size PDF output.  This fixes a reported build
failure for the 9.3beta2 A4 PDF docs, and future-proofs this particular
para against causing similar problems in future.  (Perhaps somebody will
fix this issue in the SGML/TeX documentation tool chain someday, but I'm
not holding my breath.)

Back-patch to all supported branches, since the same problem could rise up
to bite us in future updates if anyone changes anything earlier than this
in func.sgml.

doc/src/sgml/func.sgml

@@ -10205,12 +10205,11 @@ table2-mapping
   </indexterm>
 
   <para>
-   This section describes <productname>PostgreSQL</productname>'s
-   functions for operating on <firstterm>sequence objects</firstterm>.
-   Sequence objects (also called sequence generators or just
-   sequences) are special single-row tables created with <xref
+   This section describes functions for operating on <firstterm>sequence
+   objects</firstterm>, also called sequence generators or just sequences.
+   Sequence objects are special single-row tables created with <xref
    linkend="sql-createsequence">.
-   A sequence object is usually used to generate unique identifiers
+   Sequence objects are commonly used to generate unique identifiers
    for rows of a table.  The sequence functions, listed in <xref
    linkend="functions-sequence-table">, provide simple, multiuser-safe
    methods for obtaining successive sequence values from sequence