Export WebSocketDisconnect and add example handling disconnections to docs

[rkbeatss]

PR for #1799 : Adding documentation for multiple clients and what happens when a client closes WebSocket

[codecov]

Codecov Report

Merging #1822 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            master     #1822    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          235       238     +3     
  Lines         6989      7094   +105     
==========================================
+ Hits          6989      7094   +105     

Impacted Files	Coverage Δ
.../test_tutorial/test_websockets/test_tutorial003.py	100.00% <100.00%> (ø)
fastapi/params.py	100.00% <0.00%> (ø)
fastapi/routing.py	100.00% <0.00%> (ø)
fastapi/encoders.py	100.00% <0.00%> (ø)
fastapi/exceptions.py	100.00% <0.00%> (ø)
fastapi/applications.py	100.00% <0.00%> (ø)
fastapi/security/http.py	100.00% <0.00%> (ø)
fastapi/param_functions.py	100.00% <0.00%> (ø)
fastapi/security/api_key.py	100.00% <0.00%> (ø)
fastapi/dependencies/utils.py	100.00% <0.00%> (ø)
... and 8 more

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[github-actions]

📝 Docs preview for commit 0f390ed at: https://5f247b1250231f85bbed1881--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit e5d7d78 at: https://5f2488b350231f2ad0ed1fcf--fastapi.netlify.app

[github-actions]

📝 Docs preview for commit fa43455 at: https://5f2c3991865d01cf3361311f--fastapi.netlify.app

[tiangolo]

Excellent job @rkbeatss ! Thanks! 🚀 🎉 🤓 🍰

Thanks for the help @Kludex and @teymour-aldridge ! ☕ 🍫