docs: mention GLUON_RELEASE in make manifest instructions

Adding the fact that GLUON_RELEASE is needed with 'make manifest'
if it is used with 'make'.

This fact was mentioned in the v2017.1 release notes but is not (clearly)
mentioned inside other parts of the documentation.

Signed-off-by: Chrissi^ <chris@tinyhost.de>

[Matthias Schiffer: minor rewording]

docs/features/autoupdater.rst

@@ -22,6 +22,11 @@ for updates hourly (at a random minute of the hour), but usually only updates du
 
 ``GLUON_PRIORITY`` may be an integer or a decimal fraction.
 
+If ``GLUON_RELEASE`` is passed to ``make`` explicitly or it is generated dynamically
+in ``site.mk``, care must be taken to pass the same ``GLUON_RELEASE`` to ``make manifest``,
+as otherwise the generated manifest will be incomplete.
+
+
 Automated nightly builds
 ------------------------
 

docs/user/getting_started.rst

@@ -184,7 +184,8 @@ GLUON_REGION
 GLUON_RELEASE
   Firmware release number: This string is displayed in the config mode, announced
   via respondd/alfred and used by the autoupdater to decide if a newer version
-  is available.
+  is available. The same GLUON_RELEASE has to be passed to ``make`` and ``make manifest``
+  to generate a correct manifest.
 
 GLUON_TARGET
   Target architecture to build.