Note to escape exclamation mark in *nix (#15972)

* Note to escape exclamation mark in *nix

* Apply suggestions from code review

Co-Authored-By: Maira Wenzel <mairaw@microsoft.com>

* Namespace is one word

Co-Authored-By: Maira Wenzel <mairaw@microsoft.com>

Co-authored-by: Maira Wenzel <mairaw@microsoft.com>

Author: joacar

docs/core/testing/selective-unit-tests.md

@@ -9,6 +9,11 @@ ms.date: 03/22/2017
 
 With the `dotnet test` command in .NET Core, you can use a filter expression to run selective tests. This article demonstrates how to filter which test are run. The following examples use `dotnet test`. If you're using `vstest.console.exe`, replace `--filter` with `--testcasefilter:`.
 
+> [!NOTE]
+> Using filters that include exclamation mark (!) on `*nix` requires escaping since `!` is reserved. For example, this filter
+> skips all tests if the namespace contains IntegrationTests: `dotnet test --filter FullyQualifiedName\!~IntegrationTests`.
+> Note the backslash that precedes the exclamation mark.
+
 ## MSTest
 
 ```csharp