Update CONTRIBUTION.md after CI side changes (#4598)

* Update CONTRIBUTION.md after CI side changes

* Fix a typo

---------

Co-authored-by: Yong He <yonghe@outlook.com>

Author: jkwak-work

CONTRIBUTION.md

@@ -82,7 +82,6 @@ C:\git\slang> cmake.exe --preset vs2019 # For VisualStudio 2019
 ```
 
 Open `build/slang.sln` with VisualStudio IDE and build it for "x64".
-> Warning: there is another file, `slang.sln`, at the root directory. When you use cmake workflow, you must use one in the `build` directory.
 
 Or you can build with a following command:
 ```
@@ -110,9 +109,11 @@ $ cmake --build --preset release
 Make your changes and ensure to follow our [Design Decisions](docs/design/README.md).
 
 ### Testing
-Test your changes thoroughly to ensure they do not introduce new issues. This is done by building and running a slang-test from the repository root directory. For more details about slang-test, please refer to a [Documentation on testing](tools/slang-test/README.md).
+Test your changes thoroughly to ensure they do not introduce new issues. This is done by building and running a "slang-test" from the repository root directory. For more details about "slang-test", please refer to a [Documentation on testing](tools/slang-test/README.md).
 
-If you are familiar with Workflow/Actions in github, you can check [Our Workflows](.github/workflows). [Windows-selfhosted.yml](.github/workflows/windows-selfhosted.yml) is a good starting point.
+> Note: One of the options for `slang-test.exe` is `-api`, and it takes an additional keyword to specify which API to test. When the option is `-api all-cpu`, as an example, it means it tests all APIs except CPU. The minus sign (-) after `all` means "exclude" and you can "include" with plus sign (+) like `-api gl+dx11`.
+
+If you are familiar with Workflow/Actions in github, you can check [Our Workflows](.github/workflows). "Test Slang" section in [ci.yml](.github/workflows/ci.yml) is where "slang-test" runs.
 
 For a quick reference, follow the instructions below.
 
@@ -126,6 +127,7 @@ For a quick reference, follow the instructions below.
    ```
    C:\git\slang> bin\windows-x64\release\slang-test.exe -use-test-server -server-count 8
    ```
+   > Note: if you increase `-server-count` more than 16, you may find some of tests randomly fail. This is a known issue on the graphics driver side.
 1. Check whether the test is finished as expected.
 
 #### Linux