reproducible-builds: add a "How to" section with OEQA* variables

Shows an example of the new variable OEQA_REPRODUCIBLE_TEST_LEAF_TARGETS
and still mention OEQA_REPRODUCIBLE_TEST_TARGET/SSTATE_TARGETS that were
shown earlier.

(From yocto-docs rev: 78cf8b1ff1cdfbc863033f267da8fb60419b50fc)

Signed-off-by: Yoann Congal <yoann.congal@smile.fr>
Reviewed-by: Guillaume Swaenepoel <guillaume.swaenepoel@smile.fr>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/test-manual/reproducible-builds.rst

@@ -145,3 +145,23 @@ set of recipes before the test, meaning they are excluded from reproducibility
 testing. As a practical example, you could set ``sstate_targets`` to
 ``core-image-sato``, then setting ``targets`` to ``core-image-sato-sdk`` would
 run reproducibility tests only on the targets belonging only to ``core-image-sato-sdk``.
+
+Using :term:`OEQA_REPRODUCIBLE_TEST_* <OEQA_REPRODUCIBLE_TEST_LEAF_TARGETS>` variables
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+If you want to test the reproducibility of a set of recipes, you can define
+:term:`OEQA_REPRODUCIBLE_TEST_LEAF_TARGETS`, in your local.conf::
+
+   OEQA_REPRODUCIBLE_TEST_LEAF_TARGETS = "my-recipe"
+
+This will test the reproducibility of ``my-recipe`` but will use the
+:ref:`Shared State <overview-manual/concepts:Shared State>` for most its
+dependencies (i.e. the ones explicitly listed in DEPENDS, which may not be all
+dependencies, c.f. [depends] varflags, PACKAGE_DEPENDS and other
+implementations).
+
+You can have finer control on the test with:
+
+- :term:`OEQA_REPRODUCIBLE_TEST_TARGET`: lists recipes to be built,
+- :term:`OEQA_REPRODUCIBLE_TEST_SSTATE_TARGETS`: lists recipes that will
+  be built using :ref:`Shared State <overview-manual/concepts:Shared State>`.