Add xreflabels to /contrib manuals so links appear correct. Also update

README.links to explain xref properly.

Author: bmomjian

doc/src/sgml/README.links

@@ -7,8 +7,8 @@ Intra-document Linking
 ----------------------
 
 <xref>
-use to get link text from the title of the target link; has no
-close tag
+use to get chapter/section # from the title of the target
+link, or xreflabel if defined at the target; has no close tag
 http://www.oasis-open.org/docbook/documentation/reference/html/xref.html
 
 <link>
@@ -28,7 +28,8 @@ External Linking
 
 <ulink>
 like <link>, but uses a URL (not a document target); requires
-</ulink>; if no text is specified, the URL appears as the link text
+</ulink>; if no text is specified, the URL appears as the link
+text
 http://www.oasis-open.org/docbook/documentation/reference/html/ulink.html
 
 url=
@@ -40,4 +41,5 @@ Guidelines
 
 o If you want to supply text, use <link>, else <xref>
 o Do not use text with <ulink> so the URL appears in printed output
-o You can supply text at the target location with 'xreflabel'
+o Specific nouns like GUC variables, SQL commands, and contrib modules
+ usually have xreflabels

doc/src/sgml/adminpack.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/adminpack.sgml -->
 
-<sect1 id="adminpack">
+<sect1 id="adminpack" xreflabel="adminpack">
  <title>adminpack</title>
 
  <indexterm zone="adminpack">

doc/src/sgml/auth-delay.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/auth-delay.sgml -->
 
-<sect1 id="auth-delay">
+<sect1 id="auth-delay" xreflabel="auth_delay">
  <title>auth_delay</title>
 
  <indexterm zone="auth-delay">

doc/src/sgml/auto-explain.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/auto-explain.sgml -->
 
-<sect1 id="auto-explain">
+<sect1 id="auto-explain" xreflabel="auto_explain">
  <title>auto_explain</title>
 
  <indexterm zone="auto-explain">

doc/src/sgml/btree-gin.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/btree-gin.sgml -->
 
-<sect1 id="btree-gin">
+<sect1 id="btree-gin" xreflabel="btree_gin">
  <title>btree_gin</title>
 
  <indexterm zone="btree-gin">

doc/src/sgml/btree-gist.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/btree-gist.sgml -->
 
-<sect1 id="btree-gist">
+<sect1 id="btree-gist" xreflabel="btree_gist">
  <title>btree_gist</title>
 
  <indexterm zone="btree-gist">

doc/src/sgml/chkpass.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/chkpass.sgml -->
 
-<sect1 id="chkpass">
+<sect1 id="chkpass" xreflabel="chkpass">
  <title>chkpass</title>
 
  <indexterm zone="chkpass">

doc/src/sgml/citext.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/citext.sgml -->
 
-<sect1 id="citext">
+<sect1 id="citext" xreflabel="citext">
  <title>citext</title>
 
  <indexterm zone="citext">

doc/src/sgml/contrib-spi.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/contrib-spi.sgml -->
 
-<sect1 id="contrib-spi">
+<sect1 id="contrib-spi" xreflabel="spi">
  <title>spi</title>
 
  <indexterm zone="contrib-spi">

doc/src/sgml/cube.sgml

@@ -1,6 +1,6 @@
 <!-- doc/src/sgml/cube.sgml -->
 
-<sect1 id="cube">
+<sect1 id="cube" xreflabel="cube">
  <title>cube</title>
 
  <indexterm zone="cube">