Update session.md
Update session.md

Added documentation for the once function added in PR #4137.
laravel-china/laravel-docs,tusharvikky/docs,ryanwinchester/laravel-docs,iwzh/laravel-doc-offline,tusharvikky/docs,iwzh/laravel-docs,ryanwinchester/laravel-docs,chenrenzhen/laravel-docs-china,xpressengine/laravel-korean-docs,khs1994-gitbook/laravel-docs,xpressengine/laravel-korean-docs