Add documentation for numeric value comparison within specified accuracy #165 (#314)

Add documentation about how to compare two numeric values within
specified accuracy.

### Motivation:

Issue #165. swift-testing doesn't have an analog of [XCAssertEquals of
specified
accuracy](https://developer.apple.com/documentation/xctest/3551607-xctassertequal).
There is no such function in the standard library. It's unclear to users
how they can compare two numeric values.

Resolves #165. 

### Modifications:

Added a comment in the `#expect` macro: should use
`isApproximatelyEqual` in the swift-numerics.

### Result:

Clearer to users what numeric value comparison is.

### 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: wangxingyu5529

Sources/Testing/Testing.docc/MigratingFromXCTest.md

@@ -359,6 +359,11 @@ their equivalents in the testing library:
 | `try XCTUnwrap(x)` | `try #require(x)` |
 | `XCTFail("…")` | `Issue.record("…")` |
 
+The testing library doesn’t provide an equivalent of
+[`XCTAssertEqual(_:_:accuracy:_:file:line:)`](https://developer.apple.com/documentation/xctest/3551607-xctassertequal).
+To compare two numeric values within a specified accuracy, 
+use `isApproximatelyEqual()` from [swift-numerics](https://github.com/apple/swift-numerics).
+
 ### Continue or halt after test failures
 
 An instance of an `XCTestCase` subclass can set its