Add 2.1.2 release date and document experimental fluid interaction model in the changelog.

PiperOrigin-RevId: 434760829
Change-Id: I71d3ca9864890927d2945c73767511e5e973be73

doc/changelog.rst

@@ -2,8 +2,8 @@
 Changelog
 =========
 
-Version 2.1.2 (Date TBD, 2022)
-------------------------------
+Version 2.1.2 (Mar. 15, 2022)
+-----------------------------
 
 New modules
 ^^^^^^^^^^^
@@ -64,10 +64,16 @@ General
    perspective of C and C++, this is a non-change since array types in function signatures decay to pointer types.
    However, it allows autogenerated code to be aware of expected input shapes.
 
+#. Experimental stateless fluid interaction model. As described :ref:`here <gePassive>`, fluid forces use sizes computed
+   from body inertia. While sometimes convenient, this is very rarely a good approximation. In the new model forces act
+   on geoms, rather than bodies, and have a several user-settable parameters. The model is activated by setting a new
+   attribute: ``<geom fluidshape="ellipsoid"/>``. The parameters are described succinctly :ref:`here<geom>`, but we
+   leave a full description or the model and its parameters to when this feature leaves experimental status.
+
 Bug Fixes
 ^^^^^^^^^
 
-15. ``mj_loadXML`` and ``mj_saveLastXML`` are now locale-independent. The Unity plugin should now work correctly for
+16. ``mj_loadXML`` and ``mj_saveLastXML`` are now locale-independent. The Unity plugin should now work correctly for
     users whose system locales use commas as decimal separators.
 #.  XML assets in VFS no longer need to end in a null character. Instead, the file size is determined by the size
     parameter of the corresponding VFS entry.
@@ -77,7 +83,7 @@ Bug Fixes
 Binary build
 ^^^^^^^^^^^^
 
-18. Windows binaries are now built with Clang.
+20. Windows binaries are now built with Clang.
 
 Version 2.1.1 (Dec. 16, 2021)
 -----------------------------