Add docstring to InternalClock::now() method

[Lang-Qiu]

Problem Background

The public method now() in InternalClock class implements the Psr\Clock\ClockInterface but lacks a docstring. Adding documentation improves code readability and self-documentation, making it easier for developers to understand the method's purpose and behavior.

Changes Made

• Added a docstring to the now() method in src/InternalClock.php to describe its functionality: returning the current time as a DateTimeImmutable object.

Verification

• The change is minimal and only adds documentation, so it should not affect functionality.
• Run the existing test suite to ensure no regressions or failures are introduced. The code should pass all tests as before.
• Verify that the docstring style is consistent with the repository's existing conventions.