doc: Add missing <varlistentry> markups for developer GUCs

Missing such markups makes it impossible to create links back to these
GUCs, and all the other parameters have one already.

Author: Ian Lawrence Barwick
Discussion: https://postgr.es/m/CAB8KJ=jx=6dFB_EN3j0UkuvG3cPu5OmQiM-ZKRAz+fKvS+u8Ng@mail.gmail.com
Backpatch-through: 11

Author: michaelpq

doc/src/sgml/config.sgml

@@ -11099,7 +11099,7 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
       </listitem>
      </varlistentry>
 
-     <varlistentry>
+     <varlistentry id="guc-trace-locks" xreflabel="trace_locks">
       <term><varname>trace_locks</varname> (<type>boolean</type>)
       <indexterm>
        <primary><varname>trace_locks</varname> configuration parameter</primary>
@@ -11140,7 +11140,7 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
       </listitem>
      </varlistentry>
 
-     <varlistentry>
+     <varlistentry id="guc-trace-lwlocks" xreflabel="trace_lwlocks">
       <term><varname>trace_lwlocks</varname> (<type>boolean</type>)
       <indexterm>
        <primary><varname>trace_lwlocks</varname> configuration parameter</primary>
@@ -11160,7 +11160,7 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
       </listitem>
      </varlistentry>
 
-     <varlistentry>
+     <varlistentry id="guc-trace-userlocks" xreflabel="trace_userlocks">
       <term><varname>trace_userlocks</varname> (<type>boolean</type>)
       <indexterm>
        <primary><varname>trace_userlocks</varname> configuration parameter</primary>
@@ -11179,7 +11179,7 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
       </listitem>
      </varlistentry>
 
-     <varlistentry>
+     <varlistentry id="guc-trace-lock-oidmin" xreflabel="trace_lock_oidmin">
       <term><varname>trace_lock_oidmin</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>trace_lock_oidmin</varname> configuration parameter</primary>
@@ -11198,7 +11198,7 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
       </listitem>
      </varlistentry>
 
-     <varlistentry>
+     <varlistentry id="guc-trace-lock-table" xreflabel="trace_lock_table">
       <term><varname>trace_lock_table</varname> (<type>integer</type>)
       <indexterm>
        <primary><varname>trace_lock_table</varname> configuration parameter</primary>
@@ -11216,7 +11216,7 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
       </listitem>
      </varlistentry>
 
-     <varlistentry>
+     <varlistentry id="guc-debug-deadlocks" xreflabel="debug_deadlocks">
       <term><varname>debug_deadlocks</varname> (<type>boolean</type>)
       <indexterm>
        <primary><varname>debug_deadlocks</varname> configuration parameter</primary>
@@ -11235,7 +11235,7 @@ LOG:  CleanUpLock: deleting: lock(0xb7acd844) id(24688,24696,0,0,0,1)
       </listitem>
      </varlistentry>
 
-     <varlistentry>
+     <varlistentry id="guc-log-btree-build-stats" xreflabel="log_btree_build_stats">
       <term><varname>log_btree_build_stats</varname> (<type>boolean</type>)
       <indexterm>
        <primary><varname>log_btree_build_stats</varname> configuration parameter</primary>