Use 3+ as the supported NUnit version and mention migration 3 -> 4.

src/nunit.analyzers/nunit.analyzers.nuspec

@@ -12,10 +12,10 @@
     <repository type="git" url="https://github.com/nunit/nunit.analyzers"/>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>
     <developmentDependency>true</developmentDependency>
-    <summary>Code analyzers and fixes for NUnit 3</summary>
+    <summary>Code analyzers and fixes for NUnit 3+</summary>
     <readme>docs/README.md</readme>
     <description>
-This package includes analyzers and code fixes for test projects using NUnit 4. The analyzers will mark wrong usages when writing tests, and the code fixes can be used to used to correct these usages.
+This package includes analyzers and code fixes for test projects using NUnit 3+. The analyzers will mark wrong usages when writing tests, and the code fixes can be used to used to correct these usages. They will also aid in the transition from NUnit 3 to NUnit 4.
 
 Version 3.0 and upwards works in Visual Studio 2019 (version 16.3) or newer and also enables supression of compiler errors such as errors arising from nullable reference types. For Visual Studio 2017 one must use versions below 3.0 - note that these versions are no longer updated, so version 2.10.0 is the last version that works in Visual Studio 2017.
     </description>