docs(eventsource): document connection lifecycle and close usage

[nthbotast]

Summary

• add a dedicated section describing how EventSource manages long-lived text/event-stream connections
• document that .close() should be called to release connection resources when no longer needed
• add a small example with onmessage, onerror, and explicit shutdown

Closes #536

Validation

• git diff --check

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.03%. Comparing base (fb05ad6) to head (428d53a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4872   +/-   ##
=======================================
  Coverage   93.02%   93.03%           
=======================================
  Files         112      112           
  Lines       35176    35176           
=======================================
+ Hits        32723    32725    +2     
+ Misses       2453     2451    -2     

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[metcoder95]

Don't mind these kind of contributions as are not making much of a harm, but would appreciate to state that as disclosure of the PR description

[mcollina]

lgtm