family.py: bump --to asks add's duplicate-id question, peels an annotated tag's own id as add --at does, and says to push first a commit the member does not hold after its fetch (#541)

docs/cli.md

@@ -757,7 +757,8 @@ options:
   --member ORG/PROJECT  the member's ASSEMBLY ROOT, `<org>/<Project>` — never
                         a leg
   --at COMMIT           pin this 40-hex commit instead of the member's current
-                        tip
+                        tip (an annotated tag's own 40-hex id is peeled to the
+                        commit it names)
   --local-remote-dir LOCAL_REMOTE_DIR
                         resolve the member to a bare repository here instead
                         of github.com (the TEST path)
@@ -790,7 +791,8 @@ options:
   --family-root FAMILY_ROOT
   --member PROJECT
   --to COMMIT           the 40-hex commit to move the gitlink and the pin to,
-                        together
+                        together (an annotated tag's own 40-hex id is peeled
+                        to the commit it names, as `add --at` peels one)
   --trailer "KEY: VALUE"
                         append this `<Key>: <value>` line to the commit this
                         writes; repeatable, kept in the order given