Clarify AuthenticationManagerBuilder.authenticationProvider JavaDoc

[ckyeon]

Summary

• Clarifies that the authenticationProvider() method adds a provider to the existing list rather than replacing it
• Documents that the provider is appended to the internal collection
• Adds missing @param tag for the authenticationProvider parameter

Context

The original JavaDoc for AuthenticationManagerBuilder.authenticationProvider() could be misinterpreted as setting a single provider.

This change makes it explicit that the method appends the provider to the internal collection, which is important for users configuring multiple authentication providers.