Adding change log for coq#12146.

Co-Authored-By: Théo Zimmermann <theo.zimmi@gmail.com>

doc/changelog/04-tactics/12146-master+fix10812-subst-failure-section-variables.rst

@@ -0,0 +1,9 @@
+- **Changed:**
+  Tactic :tacn:`subst @id` now fails over a section variable which is
+  indirectly dependent in the goal; the incompatibility can generally
+  be fixed by first clearing the hypotheses causing an indirect
+  dependency, as reported by the error message, or by using :tacn:`rewrite`
+  instead; similarly, :tacn:`subst` has no more effect on such variables
+  (`#12146 <https://github.com/coq/coq/pull/12146>`_,
+  by Hugo Herbelin; fixes `#10812 <https://github.com/coq/coq/pull/10812>`_;
+  fixes `#12139 <https://github.com/coq/coq/pull/12139>`_).