dev-manual/building.rst: add note about externalsrc variables absolute paths

(From yocto-docs rev: 33166d70b6ad093bd95ffccafc0b07820682f656)

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

documentation/dev-manual/building.rst

@@ -757,6 +757,11 @@ to point to that directory::
 
    EXTERNALSRC_BUILD:pn-myrecipe = "path-to-your-source-tree"
 
+.. note::
+
+   The values of :term:`EXTERNALSRC` and :term:`EXTERNALSRC_BUILD`
+   must be absolute paths.
+
 Replicating a Build Offline
 ===========================