Improve documentation of PasswordUpgraderInterface

[stof]

Q	A
Branch?	7.3
Bug fix?	no
New feature?	no
Deprecations?	no
Issues	n/a
License	MIT

Implementations of PasswordUpgraderInterface can be called with a user instance that they don't support and are expected to handle it gracefully.
Handling it gracefully can be done either by being silent or by throwing an UnsupportedUserException.

This is a follow-up of #51283 to document that throwing UnsupportedUserException is actually fine for implementations as the way to handle unsupported users, as shown by the implementation of the ChainUserProvider:

symfony/src/Symfony/Component/Security/Core/User/ChainUserProvider.php

Lines 107 to 113 in 07e020a

	if ($provider instanceof PasswordUpgraderInterface) {
	try {
	$provider->upgradePassword($user, $newHashedPassword);
	} catch (UnsupportedUserException) {
	// ignore: password upgrades are opportunistic
	}
	}

[stof]

Note that the LdapUserProvider and EntityUserProvider in core are already relying on the fact that throwing an UnsupportedUserException is allowed.

[nicolas-grekas]

Thank you @stof