Note in documentation when #expect(throws:) started returning a value. (#980)

DocC doesn't let us annotate our return type's availability separately
from our macros' availability, and we can't include both the old symbols
and new ones because it is ambiguous at compile time, so this PR
explains in prose when the return types were added.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/Expectations/Expectation+Macro.swift

@@ -160,6 +160,9 @@ public macro require<T>(
 /// is running in the current task. Any value returned by `expression` is
 /// discarded.
 ///
+/// - Note: If you use this macro with a Swift compiler version lower than 6.1,
+///   it doesn't return a value.
+///
 /// If the thrown error need only equal another instance of [`Error`](https://developer.apple.com/documentation/swift/error),
 /// use ``expect(throws:_:sourceLocation:performing:)-7du1h`` instead.
 ///
@@ -224,6 +227,9 @@ public macro require<T>(
 /// is running in the current task and an instance of ``ExpectationFailedError``
 /// is thrown. Any value returned by `expression` is discarded.
 ///
+/// - Note: If you use this macro with a Swift compiler version lower than 6.1,
+///   it doesn't return a value.
+///
 /// If the thrown error need only equal another instance of [`Error`](https://developer.apple.com/documentation/swift/error),
 /// use ``require(throws:_:sourceLocation:performing:)-4djuw`` instead.
 ///
@@ -286,6 +292,9 @@ public macro require<R>(
 /// not equal to `error`, an ``Issue`` is recorded for the test that is running
 /// in the current task. Any value returned by `expression` is discarded.
 ///
+/// - Note: If you use this macro with a Swift compiler version lower than 6.1,
+///   it doesn't return a value.
+///
 /// If the thrown error need only be an instance of a particular type, use
 /// ``expect(throws:_:sourceLocation:performing:)-1hfms`` instead.
 @discardableResult
@@ -327,6 +336,9 @@ public macro require<R>(
 /// in the current task and an instance of ``ExpectationFailedError`` is thrown.
 /// Any value returned by `expression` is discarded.
 ///
+/// - Note: If you use this macro with a Swift compiler version lower than 6.1,
+///   it doesn't return a value.
+///
 /// If the thrown error need only be an instance of a particular type, use
 /// ``require(throws:_:sourceLocation:performing:)-7n34r`` instead.
 @discardableResult