fix doc to mention env setting for v0.7.3-dev (#602)

### What this PR does / why we need it?
fix doc to mention env setting for v0.7.3-dev

Signed-off-by: Shuqiao Li <celestialli@outlook.com>

Author: celestialli

docs/source/installation.md

@@ -190,11 +190,14 @@ cd vllm-ascend
 python setup.py develop
 cd ..
 ```
+
+vllm-ascend will build custom ops by default. If you don't want to build it, set `COMPILE_CUSTOM_KERNELS=0` environment to disable it.
 :::
 
 ```{note}
-vllm-ascend will build custom ops by default. If you don't want to build it, set `COMPILE_CUSTOM_KERNELS=0` environment to disable it.
+If you are building from v0.7.3-dev and intend to use sleep mode feature, you should set `COMPILE_CUSTOM_KERNELS=1` manually.
 To build custom ops, gcc/g++ higher than 8 and c++ 17 or higher is required. If you're using `pip install -e .` and encourage a torch-npu version conflict, please install with `pip install --no-build-isolation -e .` to build on system env.
+If you encounter other problems during compiling, it is probably because unexpected compiler is being used, you may export `CXX_COMPILER` and `C_COMPILER` in env to specify your g++ and gcc locations before compiling.
 ```
 
 ::::