Document the role of CredentialsContainer

Closes gh-15319

docs/modules/ROOT/nav.adoc

@@ -44,6 +44,7 @@
 ***** xref:servlet/authentication/passwords/in-memory.adoc[In Memory]
 ***** xref:servlet/authentication/passwords/jdbc.adoc[JDBC]
 ***** xref:servlet/authentication/passwords/user-details.adoc[UserDetails]
+***** xref:servlet/authentication/passwords/credentials-container.adoc[CredentialsContainer]
 ***** xref:servlet/authentication/passwords/user-details-service.adoc[UserDetailsService]
 ***** xref:servlet/authentication/passwords/password-encoder.adoc[PasswordEncoder]
 ***** xref:servlet/authentication/passwords/dao-authentication-provider.adoc[DaoAuthenticationProvider]

docs/modules/ROOT/pages/servlet/authentication/passwords/credentials-container.adoc

@@ -0,0 +1,12 @@
+[[servlet-authentication-credentialscontainer]]
+= CredentialsContainer
+
+{security-api-url}org/springframework/security/core/CredentialsContainer.html[The `CredentialsContainer`] interface indicates that the implementing object contains sensitive data, and is used internally by Spring Security to erase the authentication credentials after a successful authentication.
+This interface is implemented by most of Spring Security internal domain classes, like {security-api-url}org/springframework/security/core/userdetails/User.html[User] and {security-api-url}org/springframework/security/authentication/UsernamePasswordAuthenticationToken.html[UsernamePasswordAuthenticationToken].
+
+The `ProviderManager` manager checks whether the returned `Authentication` implements this interface.
+If so, xref:servlet/authentication/architecture.adoc#servlet-authentication-providermanager-erasing-credentials[it calls the `eraseCredentials` method] to remove the credentials from the object.
+
+If you want your custom authentication objects to have their credentials erased after authentication, you should ensure that the classes implement the `CredentialsContainer` interface.
+
+Users who are writing their own `AuthenticationProvider` implementations should create and return an appropriate `Authentication` object there, minus any sensitive data, rather than using this interface.