Document that alerts are removed from the DOM when closed; fixes #14318
bump grunt-autoprefixer to ~1.0.0
Merge pull request #14332 from hnrch02/clarify-alert-removal

Document that alerts are removed from the DOM when closed; fixes #14318
Merge pull request #14334 from twbs/update-autoprefixer

bump grunt-autoprefixer to ~1.0.0
Small wording tweak in alert docs
Clean up buttons documentation