refactor(docs): standardize Javadoc across API and improve internal documentation

[devspexx]

• Standardized Javadoc across all core API classes to be doclint-compliant
• Added @since annotations to all public and internal methods
• Improved method-level documentation with clear behavior and failure descriptions
• Introduced consistent documentation structure (Behavior, Failure Conditions, Threading where applicable)
• Added detailed inline comments for complex logic (watcher, checksum, loading flow)
• Clarified API contracts and guarantees (immutability, non-null returns, event behavior)
• Improved readability and consistency across:
  • ConfigManager
  • GlobalConfigWatcher
  • YamlConfig
  • ConfigLoadResult / ConfigLoadStatus
  • FileChecksum
  • ConfigReloadedEvent
  • ConfigurationAPI bootstrap

No functional changes. Documentation and clarity improvements only.