doc: in contrib-spi, mention and link to the meaning of SPI

Also remove outdated comment about SPI subtransactions.

Reported-by: gregory@arenius.com

Discussion: https://postgr.es/m/151726276676.1240.10501743959198501067@wrigleys.postgresql.org

Backpatch-through: 9.3

doc/src/sgml/contrib-spi.sgml

@@ -10,7 +10,9 @@
 
  <para>
   The <application>spi</application> module provides several workable examples
-  of using SPI and triggers.  While these functions are of some value in
+  of using the <link linkend="spi">Server Programming Interface</link>
+  (<acronym>SPI</acronym>) and triggers.  While these functions are of
+  some value in
   their own right, they are even more useful as examples to modify for
   your own purposes.  The functions are general enough to be used
   with any table, but you have to specify table and field names (as described

doc/src/sgml/spi.sgml

@@ -42,8 +42,7 @@
   have documented error-return conventions.  Those conventions only apply
   for errors detected within the SPI functions themselves, however.)
   It is possible to recover control after an error by establishing your own
-  subtransaction surrounding SPI calls that might fail.  This is not currently
-  documented because the mechanisms required are still in flux.
+  subtransaction surrounding SPI calls that might fail.
  </para>
 
  <para>