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

@@ -1,111 +1,111 b''
 HG
     Path to the 'hg' executable, automatically passed when running
     hooks, extensions or external tools. If unset or empty, this is
     the hg executable's name if it's frozen, or an executable named
     'hg' (with %PATHEXT% [defaulting to COM/EXE/BAT/CMD] extensions on
     Windows) is searched.
 
 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.
     This setting is used to convert data including usernames,
     changeset descriptions, tag names, and branches. This setting can
     be overridden with the --encoding command-line option.
 
 HGENCODINGMODE
     This sets Mercurial's behavior for handling unknown characters
     while transcoding user input. The default is "strict", which
     causes Mercurial to abort if it can't map a character. Other
     settings include "replace", which replaces unknown characters, and
     "ignore", which drops them. This setting can be overridden with
     the --encodingmode command-line option.
 
 HGENCODINGAMBIGUOUS
     This sets Mercurial's behavior for handling characters with
     "ambiguous" widths like accented Latin characters with East Asian
     fonts. By default, Mercurial assumes ambiguous characters are
     narrow, set this variable to "wide" if such characters cause
     formatting problems.
 
 HGMERGE
     An executable to use for resolving merge conflicts. The program
     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
     files. Item separator is ":" on Unix, ";" on Windows. If HGRCPATH
     is not set, platform default search path is used. If empty, only
     the .hg/hgrc from the current repository is read.
 
     For each element in HGRCPATH:
 
     - if it's a directory, all files ending with .rc are added
     - otherwise, the file itself will be added
 
 HGPLAIN
     When set, this disables any configuration settings that might
     change Mercurial's default output. This includes encoding,
     defaults, verbose mode, debug mode, quiet mode, tracebacks, and
     localization. This can be useful when scripting against Mercurial
     in the face of existing user configuration.
 
     Equivalent options set via command line flags or environment
     variables are not overridden.
 
 HGPLAINEXCEPT
     This is a comma-separated list of features to preserve when
     HGPLAIN is enabled. Currently the following values are supported:
 
     ``alias``
         Don't remove aliases.
     ``i18n``
         Preserve internationalization.
     ``revsetalias``
         Don't remove revset aliases.
     ``templatealias``
         Don't remove template aliases.
     ``progress``
         Don't hide progress output.
 
     Setting HGPLAINEXCEPT to anything (even an empty string) will
     enable plain mode.
 
 HGUSER
     This is the string used as the author of a commit. If not set,
     available values will be considered in this order:
 
     - HGUSER (deprecated)
     - configuration files from the HGRCPATH
     - EMAIL
     - 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.
 
 LOGNAME
     May be used as the author of a commit; see HGUSER.
 
 VISUAL
     This is the name of the editor to use when committing. See EDITOR.
 
 EDITOR
     Sometimes Mercurial needs to open a text file in an editor for a
     user to modify, for example when writing commit messages. The
     editor it uses is determined by looking at the environment
     variables HGEDITOR, VISUAL and EDITOR, in that order. The first
     non-empty one is chosen. If all of them are empty, the editor
     defaults to 'vi'.
 
 PYTHONPATH
     This is used by Python to find imported modules and may need to be
     set appropriately if this Mercurial is not installed system-wide.