Enhance engine modes documentation with Reflection mode details (#3030)

Author: AdmiralSnyder

docs/docs/execution/engine-modes.md

@@ -11,6 +11,8 @@ TUnit can run in two modes:
 
 When publishing with Native AOT, the source generation mode provides additional performance benefits and full AOT compatibility.
 
+The Reflection mode allows discovery of test cases that have been source generated by something other than TUnit, though, for example .razor files in bUnit.
+
 ## Source Generation Mode (Default)
 
 By default, TUnit always uses source generation for test discovery and execution:
@@ -36,6 +38,8 @@ Enable reflection mode by running:
 dotnet test -- --reflection
 ```
 
+Alternatively, setting the environment variable `TUNIT_EXECUTION_MODE` to `reflection` enables the reflection engine mode globally.
+
 ## Native AOT Support
 
 When publishing with Native AOT, TUnit's source generation mode provides additional benefits: