doc: add SHELL_VERBOSITY to CLI (composer#12473)

Author: alexislefebvre

doc/03-cli.md

@@ -1346,4 +1346,13 @@ If set to `1`, it is the equivalent of passing the `--with-dependencies` option
 If set to `1`, it is the equivalent of passing the `--with-all-dependencies` option to
 `update`, `require` or `remove`.
 
+### SHELL_VERBOSITY
+
+Since Composer uses [symfony/console](https://github.com/symfony/console),
+you can define the [verbosity level](https://symfony.com/doc/current/console/verbosity.html).
+`SHELL_VERBOSITY=-1` to hide the output of Composer
+(this is equivalent to using the CLI option `--quiet`).
+Please note that this will apply to every tool that rely on `symfony/console`,
+you can set `SHELL_VERBOSITY=0` after the calls to Composer in order to restore the default verbosity level.
+
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