Update docs

Author: lululxvi

README.md

@@ -54,10 +54,7 @@ All the components of DeepXDE are loosely coupled, and thus DeepXDE is well-stru
 
 ## Installation
 
-DeepXDE requires [TensorFlow](https://www.tensorflow.org/) to be installed. Both TensorFlow 1 and TensorFlow 2 can be used as the DeepXDE backend, but TensorFlow 1 is recommended:
-
-- In my tests TensorFlow 2 is 2x~3x slower than TensorFlow 1;
-- Currently L-BFGS optimizer is not supported in DeepXDE yet when using TensorFlow 2.
+DeepXDE requires [TensorFlow](https://www.tensorflow.org/) to be installed. Both TensorFlow 1 and TensorFlow 2 can be used as the DeepXDE backend, but TensorFlow 1 is recommended, because in my tests TensorFlow 2 is 2x~3x slower than TensorFlow 1.
 
 Then, you can install DeepXDE itself. If you use TensorFlow 2, you need to install DeepXDE by cloning the folder. If you use Python 2, you need to install DeepXDE using `pip`.
 

docs/user/installation.rst

@@ -1,10 +1,7 @@
 Installation
 ============
 
-DeepXDE requires `TensorFlow <https://www.tensorflow.org/>`_ to be installed. Both TensorFlow 1 and TensorFlow 2 can be used as the DeepXDE backend, but TensorFlow 1 is recommended:
-
-- In my tests TensorFlow 2 is 2x~3x slower than TensorFlow 1;
-- Currently L-BFGS optimizer is not supported in DeepXDE yet when using TensorFlow 2.
+DeepXDE requires `TensorFlow <https://www.tensorflow.org/>`_ to be installed. Both TensorFlow 1 and TensorFlow 2 can be used as the DeepXDE backend, but TensorFlow 1 is recommended, because in my tests TensorFlow 2 is 2x~3x slower than TensorFlow 1.
 
 Then, you can install DeepXDE itself. If you use TensorFlow 2, you need to install DeepXDE by cloning the folder.  If you use Python 2, you need to install DeepXDE using ``pip``.