Add title for kernel_hint_design.md & kernel_selection.md

doc/fluid/design/muti_devices/kernel_hint_design.md

@@ -1,4 +1,6 @@
-# Problem
+# Kernel Hint Design
+
+## Problem
 In PaddlePaddle's [Design](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/switch_kernel.md), one Operator may have multiple kernels. Users may have some personal preference to choose a certain type of kernel for an operator, such as `force_cpu` to choose a CPU kernel, `use_cudnn` to choose a CUDNN kernel, we need to provide a way for users to do this.
 
 In the current design, we use KernelType to describe one kernel.

doc/fluid/design/muti_devices/kernel_selection.md

@@ -1,4 +1,6 @@
-# Background
+# Kernel Selection
+
+## Background
 Every operator has many kernels because there are multiple data types, places, data layout, library type that Fluid supports. We use the `OpKernelType ` to describe kernel types that operators can hold.
 
 The `OpKernelType ` is as follows: