advice: recommend GIT_ADVICE=0 for tools

The GIT_ADVICE environment variable was added implicitly in b79deeb
(advice: add --no-advice global option, 2024-05-03) but was not
documented. Add documentation to show that it is an option for tools
that want to disable these messages. Make note that while the
--no-advice option exists, older Git versions will fail to parse that
option. The environment variable presents a way to change the behavior
of Git versions that understand it without disrupting older versions.

Signed-off-by: Derrick Stolee <stolee@gmail.com>

Documentation/config/advice.txt

@@ -1,8 +1,12 @@
 advice.*::
  These variables control various optional help messages designed to
- aid new users. When left unconfigured, Git will give the message
- alongside instructions on how to squelch it. You can tell Git
- that you do not need the help message by setting these to `false`:
+ aid new users. These are output to `stderr` by default as they are
+ intended to help human readers. Tools that execute Git as a subprocess
+ can disable these messages by setting `GIT_ACVICE=0` in the environment.
++ 
+When left unconfigured, Git will give the message alongside instructions on how
+to squelch it. You can tell Git that you do not need the help message by
+setting these to `false`:
 +
 --
  addEmbeddedRepo::

Documentation/git.txt

@@ -1027,6 +1027,17 @@ standard output.
  adequate and support for it is likely to be removed in the
  foreseeable future (along with the variable).
 
+`GIT_ADVICE`::
+ If set to `0`, then disable all advice messages. These messages are
+ intended to provide hints to human users that may help them get out of
+ problematic situations or take advantage of new features. Users can
+ disable individual messages using the `advice.*` config keys. These
+ messages may be disruptive to tools that execute Git processes, so this
+ variable is available to disable the messages. (The `--no-advice`
+ global option is also available, but old Git versions may fail when
+ this option is not understood. The environment variable will be ignored
+ by Git versions that do not understand it.)
+
 Discussion[[Discussion]]
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