Edit the documentation of SessionProvider, AbstractAuthenticationProvider::init

The documentation of SessionProvider::init says that the method can be used in implementation tests.
Now all tests must use SessionProviderTestTrait::initProvider in order to initialize a SessionProvider.
So the entire description after @internal was rewritten. Also the
similar note was added to the documentation of
AbstractAuthenticationProvider::init.

Bug: T283931
Change-Id: I1cde6fd12666ef3a7c9b1e33a718592c11cf9e29

includes/auth/AbstractAuthenticationProvider.php

@@ -56,7 +56,9 @@ abstract class AbstractAuthenticationProvider implements AuthenticationProvider
 	 * Initialise with dependencies of an AuthenticationProvider
 	 *
 	 * @since 1.37
-	 * @internal
+	 * @internal In production code AuthManager will initialize the
+	 * AbstractAuthenticationProvider, in tests
+	 * AuthenticationProviderTestTrait must be used.
 	 *
 	 * @param LoggerInterface $logger
 	 * @param AuthManager $manager

includes/session/SessionProvider.php

@@ -114,7 +114,8 @@ abstract class SessionProvider implements SessionProviderInterface {
 	 * Initialise with dependencies of a SessionProvider
 	 *
 	 * @since 1.37
-	 * @internal but can be called in implementation tests
+	 * @internal In production code SessionManager will initialize the
+	 * SessionProvider, in tests SessionProviderTestTrait must be used.
 	 *
 	 * @param LoggerInterface $logger
 	 * @param Config $config