Fix docs for keep_fargs
Fix docs for keep_fargs

Compression options `keep_fargs` and `unsafe` were decoupled in v.2.5.0 (commit 5fd1245), so document actual keep_fargs default.
davidbau/UglifyJS2,rvanvelzen/UglifyJS2,hychen/UglifyJS2,dabretin/UglifyJS2,butchi/decodecodeJS,avdg/UglifyJS2,butchi/codedecodeJS