doc: Clean up title case use

doc/src/sgml/ref/pg_rewind.sgml

@@ -103,7 +103,7 @@ PostgreSQL documentation
   </para>
 
   <warning>
-   <title>Warning: Failures while Rewinding</title>
+   <title>Warning: Failures While Rewinding</title>
    <para>
     If <application>pg_rewind</application> fails while processing, then
     the data folder of the target is likely not in a state that can be

doc/src/sgml/ref/select.sgml

@@ -2044,7 +2044,7 @@ SELECT 2+2;
   </refsect2>
 
   <refsect2>
-   <title>Omitting sub-<command>SELECT</command> aliases in <literal>FROM</literal></title>
+   <title>Omitting Sub-<command>SELECT</command> Aliases in <literal>FROM</literal></title>
 
    <para>
     According to the SQL standard, a sub-<command>SELECT</command> in the

doc/src/sgml/regress.sgml

@@ -806,7 +806,7 @@ make check PROVE_TESTS='t/001_test1.pl t/003_test3.pl'
    </para>
 
   <sect2 id="regress-tap-vars">
-   <title>Environment variables</title>
+   <title>Environment Variables</title>
 
    <para>
     Data directories are named according to the test filename, and will be