flatpak-{mask, update, remote-info}: add page (#14853)

pages/linux/flatpak-update.md

@@ -0,0 +1,16 @@
+# flatpak update
+
+> Update flatpak applications and runtimes.
+> More information: <https://docs.flatpak.org/en/latest/flatpak-command-reference.html#flatpak-update>.
+
+- Update all installed applications and runtimes (use `-y` to confirm all prompts automatically):
+
+`flatpak update`
+
+- Update only a specific app:
+
+`flatpak update {{com.example.app}}`
+
+- Update/Downgrade to a specific commit (also see flatpak remote-info and flatpak mask):
+
+`flatpak update --commit={{COMMIT}} {{com.example.app}}`