Harmonize and enhance Javadoc comments across entire codebase

[landawn]

This commit comprehensively improves Javadoc documentation for all public methods in the async-http-client library, ensuring consistency, clarity, and accuracy throughout the codebase.

Key improvements:

• Enhanced class-level documentation with comprehensive descriptions
• Added usage examples using standardized {@code} block format
• Harmonized method documentation across similar classes
• Improved @param, @return, and @throws tags for clarity
• Added cross-references with proper @link and @see tags
• Fixed typos and corrected technical inaccuracies
• Documented thread-safety and lifecycle characteristics
• Added missing copyright header to DefaultKeepAliveStrategy.java

Files updated (165 files, +8,236 lines, -764 lines):

• Core API: AsyncHttpClient, Request, Response, handlers
• Netty implementation: handlers, channels, request processing
• Request body: generators, multipart, reactive streams
• Configuration: filters, cookies, exceptions
• Authentication: OAuth, NTLM, SPNEGO
• Proxy and WebSocket support
• Utility classes and helpers
• All extras modules: guava, rxjava, rxjava2, simple, registry, etc.

All documentation now follows consistent patterns:

• Clear behavior descriptions in present tense
• Complete parameter and return value documentation
• Practical usage examples where appropriate
• RFC and specification references where applicable
• Consistent terminology and formatting

This enhancement significantly improves the developer experience and maintainability of the async-http-client library.

[hyperxpro]

Hi @landawn

I have to say I am quite impressed by the PR. I am not sure if it's LLM-generated or if you spent actual time on this, but it looks like you did the change for AHC 2.x version (which is EoL), instead of 3.x version.

Also, can you please break this down into multiple PRs (per package ideally) so it is easier to review?

Thanks!