Don't mention $FLATPAK_RUN_DIR in docs

We only want the run dir to be overridable in unit tests because we
depend on it being /run/flatpak in flatpak-create-sideload-symlinks.sh,
so don't mention it in the flatpak man page.

common/flatpak-dir.c

@@ -283,6 +283,10 @@ get_run_dir_location (void)
   if (g_once_init_enter (&path))
     {
       gsize setup_value = 0;
+      /* Note: $FLATPAK_RUN_DIR should only be set in the unit tests. At
+       * runtime, /run/flatpak is assumed by
+       * flatpak-create-sideload-symlinks.sh
+       */
       const char *config_dir = g_getenv ("FLATPAK_RUN_DIR");
       if (config_dir != NULL)
         setup_value = (gsize) config_dir;

doc/flatpak.xml

@@ -96,10 +96,9 @@
             in the <filename>sideload-repos</filename>  subdirectory of the installation directory (i.e. typically
             <filename>/var/lib/flatpak/sideload-repos</filename>). Additionally
             symlinks can be created in <filename>/run/flatpak/sideload-repos</filename>
-            (overridable by <envar>FLATPAK_RUN_DIR</envar>) which is a better location
-            for non-persistent sources (as it is cleared on reboot). These symlinks can point to either the
-            directory given to <command>flatpak create-usb</command> which by default writes to the subpath
-            <filename>.ostree/repo</filename>, or directly to an ostree repo.
+            which is a better location for non-persistent sources (as it is cleared on reboot). These symlinks
+            can point to either the directory given to <command>flatpak create-usb</command> which by default
+            writes to the subpath <filename>.ostree/repo</filename>, or directly to an ostree repo.
         </para>
 
     </refsect1>