[XCTAssert] Specify where XCTAssert may be used

The XCTAssert* family of functions require that XCTCurrentTestCase is
set. Failures that occur outside of these conditions are never reported.

Author: modocache

XCTest/XCTAssert.swift

@@ -14,6 +14,11 @@
 /// are implemented in terms of this. This function emits a test failure if the
 /// general Bool expression passed to it evaluates to false.
 ///
+/// - Requires: This and all other XCTAssert* functions must be called from
+///   within a test method, as indicated by `XCTestCaseProvider.allTests`.
+///   Assertion failures that occur outside of a test method will *not* be
+///   reported as failures.
+///
 /// - Parameter expression: A boolean test. If it evaluates to false, the
 ///   assertion fails and emits a test failure.
 /// - Parameter message: An optional message to use in the failure if the