Mention clock interoperability on docs too

Signed-off-by: Luís Cobucci <[email protected]>

docs/configuration.md

@@ -200,7 +200,8 @@ It configures which are the base constraints to be used during validation.
 <?php
 declare(strict_types=1);
 
-use Lcobucci\Clock\SystemClock;
+use Lcobucci\Clock\SystemClock; // If you prefer, other PSR-20 implementations may also be used
+                                // (https://packagist.org/providers/psr/clock-implementation)
 use Lcobucci\JWT\Configuration;
 use Lcobucci\JWT\Signer;
 use Lcobucci\JWT\Signer\Key\InMemory;

docs/quick-start.md

@@ -64,7 +64,8 @@ namespace MyApp;
 require 'vendor/autoload.php';
 
 use DateTimeImmutable;
-use Lcobucci\Clock\FrozenClock;
+use Lcobucci\Clock\FrozenClock; // If you prefer, other PSR-20 implementations may also be used
+                                // (https://packagist.org/providers/psr/clock-implementation)
 use Lcobucci\JWT\Builder;
 use Lcobucci\JWT\JwtFacade;
 use Lcobucci\JWT\Signer\Hmac\Sha256;
@@ -104,7 +105,8 @@ namespace MyApp;
 require 'vendor/autoload.php';
 
 use DateTimeImmutable;
-use Lcobucci\Clock\FrozenClock;
+use Lcobucci\Clock\FrozenClock; // If you prefer, other PSR-20 implementations may also be used
+                                // (https://packagist.org/providers/psr/clock-implementation)
 use Lcobucci\JWT\JwtFacade;
 use Lcobucci\JWT\Signer\Hmac\Sha256;
 use Lcobucci\JWT\Signer\Key\InMemory;

docs/upgrading.md

@@ -121,6 +121,16 @@ Or:
  );
 ```
 
+### `lcobucci/clock` is not installed by default anymore
+
+Thanks to [PSR-20](https://www.php-fig.org/psr/psr-20/), users can more easily plug-in other [clock implementations](https://packagist.org/providers/psr/clock-implementation) if they choose to do so.
+
+If you like and were already using `lcobucci/clock` on your system, you're required to explicitly add it as a production dependency:
+
+```sh
+composer require lcobucci/clock
+```
+
 ## v3.x to v4.x
 
 The `v4.0.0` aggregates about 5 years of work and contains **several BC-breaks**.