help: call out specific replacement configuration settings

As an aside, I'm having trouble parsing the help text meaning for HG when it is
unset or empty.  How can it be the frozen name or searched if it is empty?

mercurial/help/environment.txt

@@ -8,7 +8,7 b' HG'
 HGEDITOR
     This is the name of the editor to run when committing. See EDITOR.
 
-    (deprecated, use configuration file)
+    (deprecated, see :hg:`help config.ui.editor`)
 
 HGENCODING
     This overrides the default locale setting detected by Mercurial.
@@ -36,7 +36,7 b' HGMERGE'
     will be executed with three arguments: local file, remote file,
     ancestor file.
 
-    (deprecated, use configuration file)
+    (deprecated, see :hg:`help config.ui.merge`)
 
 HGRCPATH
     A list of files or directories to search for configuration
@@ -87,7 +87,7 b' HGUSER'
     - interactive prompt
     - LOGNAME (with ``@hostname`` appended)
 
-    (deprecated, use configuration file)
+    (deprecated, see :hg:`help config.ui.username`)
 
 EMAIL
     May be used as the author of a commit; see HGUSER.