[doc] Update sparse computation doc (#2302)

docs/sparse.rst

@@ -1,17 +1,11 @@
 .. _sparse:
 
-Sparse computation (WIP)
-========================
+Sparse computation
+==================
 
-.. warning::
+The LLVM backends (CPU/CUDA) offer full functionality of spatially sparse computation in Taichi.
 
-  The Taichi compiler backend is under migration from source-to-source compilation to LLVM for compilation speed and portability.
-  Sparse computation with the new LLVM backend is not yet fully implemented on multithreaded CPUs and GPUs.
+Please read our `paper <https://yuanming.taichi.graphics/publication/2019-taichi/taichi-lang.pdf>`_, watch the `introduction video <https://www.youtube.com/watch?v=wKw8LMF3Djo>`_, or check out
+the SIGGRAPH Asia 2019 `slides <https://yuanming.taichi.graphics/publication/2019-taichi/taichi-lang-slides.pdf>`_ for more details on sparse computation.
 
-  If you are interested in sparse computation in Taichi, please read our `paper <http://taichi.graphics/wp-content/uploads/2019/09/taichi_lang.pdf>`_, watch the `introduction video <https://www.youtube.com/watch?v=wKw8LMF3Djo>`_, or check out
-  the SIGGRAPH Asia 2019 `slides <http://taichi.graphics/wp-content/uploads/2019/12/taichi_slides.pdf>`_.
-
-  The legacy source-to-source backend (commit ``dc162e11``) provides full sparse computation functionality. However, since little engineering has been done to make that commit portable (i.e. easy to compile on different platforms),
-  we suggest waiting until the LLVM version of sparse computation is fully implemented.
-
-  Sparse computation functionalities with the new LLVM backend will be back online by the end of December 2019.
+`Taichi elements <https://github.com/taichi-dev/taichi_elements>`_ implements an high-performance MLS-MPM solver on Taichi's sparse grids.