ref-manual/structure.rst: update with info on PERSISTENT_DIR

Since we mention that SSTATE_DIR can be shared for multiple builds, also
mention that PERSISTENT_DIR should be shared alongside SSTATE_DIR.

[YOCTO #15921]

(From yocto-docs rev: 741aa29898dc7f34ebd423ff7565334b2c89e18c)

Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/structure.rst

@@ -246,7 +246,8 @@ section in the Yocto Project Development Tasks Manual.
 ----------------
 
 This directory contains several internal files used by the OpenEmbedded
-build system.
+build system. The path to this directory is defined by the
+:term:`PERSISTENT_DIR` variable.
 
 It also contains ``sanity_info``, a text file keeping track of important
 build information such as the values of :term:`TMPDIR`, :term:`SSTATE_DIR`,
@@ -364,6 +365,13 @@ directory for multiple builds or move the directory to another location.
 You can control the location of this directory through the
 :term:`SSTATE_DIR` variable.
 
+.. note::
+
+   If you are sharing the :term:`SSTATE_DIR` directory for multiple builds and
+   you are using a :ref:`overview-manual/concepts:Hash Equivalence` server with
+   :term:`BB_HASHSERVE` you should also share the :term:`PERSISTENT_DIR`
+   directory. See :ref:`structure-build-cache` below.
+
 .. _structure-build-tmp:
 
 ``build/tmp/``
@@ -403,6 +411,10 @@ modification time (mtime) changes and hashing of file contents. If no
 changes to the file are detected, then the parsed result stored in the
 cache is reused. If the file has changed, it is reparsed.
 
+This directory and the :term:`SSTATE_DIR` directory can be shared for multiple
+builds when using a :ref:`overview-manual/concepts:Hash Equivalence` server
+with :term:`BB_HASHSERVE`.
+
 .. _structure-build-tmp-deploy:
 
 ``build/tmp/deploy/``