Mention that Flathub flatpak-builder may contain downstream changes

Closes #599

Author: bbhtt

docs/building-introduction.rst

@@ -21,7 +21,8 @@ at the same time, which get bundled into the build.
 
 It is packaged by several distributions and there is also a
 `Flatpak package <https://flathub.org/apps/org.flatpak.Builder>`_ called
-``org.flatpak.Builder`` available for it on Flathub.
+``org.flatpak.Builder`` available for it on Flathub (this may contain
+Flathub specific downstream modifications).
 
 The input to ``flatpak-builder`` is a manifest file. This specifies the
 parameters for the application that will be built, such as its name and

docs/first-build.rst

@@ -16,7 +16,7 @@ popular distributions.
 
 Please also install ``flatpak-builder`` from your distribution. There is
 also a flatpak-builder `Flatpak package <https://flathub.org/apps/org.flatpak.Builder>`_
-on Flathub.
+on Flathub (this may contain Flathub specific downstream modifications).
 
 Then install the Flathub repository `userwide`::
 

docs/flatpak-builder.rst

@@ -4,7 +4,7 @@ Flatpak Builder
 ``flatpak-builder`` has already been introduced in :doc:`first-build`
 and :doc:`building-introduction`. It is packaged by most of the popular
 distributions and there is also a flatpak-builder `Flatpak package <https://flathub.org/apps/org.flatpak.Builder>`_
-on Flathub.
+on Flathub (this may contain Flathub specific downstream modifications).
 
 This page provides additional details on how to use ``flatpak-builder``,
 including the various command options that are available.