Help: Clarify CLICOLOR and CLICOLOR_FORCE environment variables

Document variable precedence.

Link to third-party documentation of the convention these follow.
For now, link to an older view of the documentation since our
implementation has not been updated for `NO_COLOR`.

Issue: #26458

Help/envvar/CLICOLOR.rst

@@ -5,10 +5,15 @@ CLICOLOR
 
 .. include:: ENV_VAR.txt
 
-Set to ``0`` to tell CMake command-line tools not to print color
+Set to ``0`` to tell command-line tools not to print color
 messages even if connected to a terminal.
+This is a `common convention`_ among command-line tools in general.
 
 See also the :envvar:`CLICOLOR_FORCE` environment variable.
+:envvar:`CLICOLOR_FORCE`, if activated, takes precedence over
+:envvar:`!CLICOLOR`.
 
 See the :variable:`CMAKE_COLOR_DIAGNOSTICS` variable to control
 color in a generated build system.
+
+.. _`common convention`: https://web.archive.org/web/20230417221418/https://bixense.com/clicolors/

Help/envvar/CLICOLOR_FORCE.rst

@@ -5,10 +5,15 @@ CLICOLOR_FORCE
 
 .. include:: ENV_VAR.txt
 
-Set to a non-empty value, other than ``0``, to tell CMake command-line
+Set to a non-empty value, other than ``0``, to tell command-line
 tools to print color messages even if not connected to a terminal.
+This is a `common convention`_ among command-line tools in general.
 
 See also the :envvar:`CLICOLOR` environment variable.
+:envvar:`!CLICOLOR_FORCE`, if activated, takes precedence over
+:envvar:`CLICOLOR`.
 
 See the :variable:`CMAKE_COLOR_DIAGNOSTICS` variable to control
 color in a generated build system.
+
+.. _`common convention`: https://web.archive.org/web/20230417221418/https://bixense.com/clicolors/