docs(web-api): note the chat stream buffer size default

[zimeg]

Summary

This PR follows slackapi/bolt-js#2696 to note the WebClient#chatStream default buffer size.

Note

• Planning to mirror this change to the Python SDK if the wording is alright! 🐍
• Was unsure if this should be included in a different example? I left this unchanged for now 📚

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.01%. Comparing base (16bd88e) to head (6e96b17).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2418   +/-   ##
=======================================
  Coverage   93.01%   93.01%           
=======================================
  Files          40       40           
  Lines       11111    11113    +2     
  Branches      713      713           
=======================================
+ Hits        10335    10337    +2     
  Misses        764      764           
  Partials       12       12           

Flag	Coverage Δ
cli-hooks	95.23% <ø> (ø)
cli-test	94.79% <ø> (ø)
oauth	77.39% <ø> (ø)
socket-mode	61.87% <ø> (ø)
web-api	98.07% <100.00%> (+<0.01%)	⬆️
webhook	96.66% <ø> (ø)

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[mwbrooks]

✅ Thanks for keeping the docs helpful and accurate! 📚

[mwbrooks]

note: Just thinking aloud - perhaps we should have a timer to flush the buffer in case 256 character isn't sent within a certain time limit 🤔 Adds complexity and bug-risk, but could be a good feature for people who are struggling with the buffer size.