[branding] Update branding.desc with new os-release feature

- Point to supported keys on the fd.o site
 - Point out need for quoting
 - Update example with "@{NAME}" for the window-title

src/branding/default/branding.desc

@@ -40,8 +40,18 @@ windowSize: 800px,520px
 #
 # bootloaderEntryName is how this installation / distro is named
 # in the boot loader (e.g. in the GRUB menu).
+#
+# These strings support substitution from /etc/os-release
+# if KDE Frameworks 5.58 are available at build-time. When
+# enabled, @{var-name} is replaced by the equivalent value
+# from os-release. All the supported var-names are in all-caps,
+# and are listed on the FreeDesktop.org site,
+#       https://www.freedesktop.org/software/systemd/man/os-release.html
+# Note that ANSI_COLOR and CPE_NAME don't make sense here, and
+# are not supported (the rest are). Remember to quote the string
+# if it contains substitutions, or you'll get YAML exceptions.
 strings:
-    productName:         Generic GNU/Linux
+    productName:         "@{NAME}"
     shortProductName:    Generic
     version:             2017.8 LTS
     shortVersion:        2017.8