Fix docs stating incorrect default for coverage output format (#3936)

* Initial plan

* Fix docs stating incorrect default for coverage output format

The documentation incorrectly stated that Cobertura is the default coverage
output format. However, .NET 10 produces binary .coverage files by default
if the --coverage-output-format flag is not supplied.

Fixes documentation in migration guides (xunit, nunit, mstest), extensions,
and troubleshooting files.

Co-authored-by: thomhurst <30480171+thomhurst@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: thomhurst <30480171+thomhurst@users.noreply.github.com>

Author: Copilot

docs/docs/extensions/extensions.md

@@ -22,7 +22,7 @@ dotnet run --configuration Release --coverage
 # Specify output location
 dotnet run --configuration Release --coverage --coverage-output ./coverage/
 
-# Specify output format (cobertura is default)
+# Specify output format (default is binary .coverage file)
 dotnet run --configuration Release --coverage --coverage-output-format cobertura
 
 # Multiple formats

docs/docs/migration/mstest.md

@@ -1013,7 +1013,7 @@ dotnet run --configuration Release --coverage
 # Specify output location
 dotnet run --configuration Release --coverage --coverage-output ./coverage/
 
-# Specify coverage format (default is cobertura)
+# Specify coverage format (default is binary .coverage file)
 dotnet run --configuration Release --coverage --coverage-output-format cobertura
 
 # Multiple formats
@@ -1055,7 +1055,7 @@ If you have CI/CD pipelines that reference Coverlet, update them to use the new
 The Microsoft coverage tool supports multiple output formats:
 
 ```bash
-# Cobertura (default, widely supported)
+# Cobertura (widely supported)
 dotnet run --configuration Release --coverage --coverage-output-format cobertura
 
 # XML (Visual Studio format)

docs/docs/migration/nunit.md

@@ -746,7 +746,7 @@ dotnet run --configuration Release --coverage
 # Specify output location
 dotnet run --configuration Release --coverage --coverage-output ./coverage/
 
-# Specify coverage format (default is cobertura)
+# Specify coverage format (default is binary .coverage file)
 dotnet run --configuration Release --coverage --coverage-output-format cobertura
 
 # Multiple formats
@@ -788,7 +788,7 @@ If you have CI/CD pipelines that reference Coverlet, update them to use the new
 The Microsoft coverage tool supports multiple output formats:
 
 ```bash
-# Cobertura (default, widely supported)
+# Cobertura (widely supported)
 dotnet run --configuration Release --coverage --coverage-output-format cobertura
 
 # XML (Visual Studio format)

docs/docs/migration/xunit.md

@@ -1054,7 +1054,7 @@ dotnet run --configuration Release --coverage
 # Specify output location
 dotnet run --configuration Release --coverage --coverage-output ./coverage/
 
-# Specify coverage format (default is cobertura)
+# Specify coverage format (default is binary .coverage file)
 dotnet run --configuration Release --coverage --coverage-output-format cobertura
 
 # Multiple formats
@@ -1096,7 +1096,7 @@ If you have CI/CD pipelines that reference Coverlet, update them to use the new
 The Microsoft coverage tool supports multiple output formats:
 
 ```bash
-# Cobertura (default, widely supported)
+# Cobertura (widely supported)
 dotnet run --configuration Release --coverage --coverage-output-format cobertura
 
 # XML (Visual Studio format)

docs/docs/troubleshooting.md

@@ -2240,7 +2240,7 @@ dotnet run --configuration Release --coverage --coverage-settings coverage.runse
 
 #### 1. Check Output Format
 ```bash
-# Default is Cobertura (widely supported)
+# Cobertura (widely supported)
 dotnet run --configuration Release --coverage --coverage-output-format cobertura
 
 # For Visual Studio