manual: mention LIBFOO_EXTRACT_CMDS

[Samuel: add precision about non-tar archive]
[Thomas: reword with more details. Fix commit title]
Signed-off-by: Stephan Hoffmann <sho@relinux.de>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-generic.txt

@@ -358,6 +358,13 @@ LIBFOO_VERSION = 2.32
 Now, the variables that define what should be performed at the
 different steps of the build process.
 
+* +LIBFOO_EXTRACT_CMDS+ lists the actions to be performed to extract
+  the package. This is generally not needed as tarballs are
+  automatically handled by Buildroot. However, if the package uses a
+  non-standard archive format, such as a ZIP or RAR file, or has a
+  tarball with a non-standard organization, this variable allows to
+  override the package infrastructure default behavior.
+
 * +LIBFOO_CONFIGURE_CMDS+ lists the actions to be performed to
   configure the package before its compilation.