ref-manual: document SSTATE_EXCLUDEDEPS_SYSROOT

Backport from master:
https://git.yoctoproject.org/yocto-docs/commit/?id=b6690011c14ce4bf30571f045152a9d324ad5039

(From yocto-docs rev: 0b0509d14742ad1cd050a79164718e6e17020c65)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/variables.rst

@@ -7335,6 +7335,32 @@ system and gives an overview of their function and contents.
    :term:`SSTATE_DIR`
       The directory for the shared state cache.
 
+   :term:`SSTATE_EXCLUDEDEPS_SYSROOT`
+      This variable allows to specify indirect dependencies to exclude
+      from sysroots, for example to avoid the situations when a dependency on
+      any ``-native`` recipe will pull in all dependencies of that recipe
+      in the recipe sysroot. This behaviour might not always be wanted,
+      for example when that ``-native`` recipe depends on build tools
+      that are not relevant for the current recipe.
+
+      This way, irrelevant dependencies are ignored, which could have
+      prevented the reuse of prebuilt artifacts stored in the Shared
+      State Cache.
+
+      :term:`SSTATE_EXCLUDEDEPS_SYSROOT` is evaluated as two regular
+      expressions of recipe and dependency to ignore. An example
+      is the rule in :oe_git:`meta/conf/layer.conf </openembedded-core/tree/meta/conf/layer.conf>`::
+
+         # Nothing needs to depend on libc-initial
+         # base-passwd/shadow-sysroot don't need their dependencies
+         SSTATE_EXCLUDEDEPS_SYSROOT += "\
+             .*->.*-initial.* \
+             .*(base-passwd|shadow-sysroot)->.* \
+         "
+
+      The ``->`` substring represents the dependency between
+      the two regular expressions.
+
    :term:`SSTATE_MIRROR_ALLOW_NETWORK`
       If set to "1", allows fetches from mirrors that are specified in
       :term:`SSTATE_MIRRORS` to work even when