doc(library/module): document the persistent set_option discussion
doc(library/module): document the persistent set_option discussion

cc @kha

dselsam/lean,leanprover/lean,leanprover-community/lean,digama0/lean,leanprover/lean,leanprover-community/lean,Kha/lean,Kha/lean,Kha/lean,fpvandoorn/lean,dselsam/lean,fpvandoorn/lean,digama0/lean,dselsam/lean,fpvandoorn/lean,fpvandoorn/lean,digama0/lean,Kha/lean,digama0/lean,Kha/lean,leanprover/lean,leanprover-community/lean,digama0/lean,fpvandoorn/lean,fpvandoorn/lean,leanprover-community/lean,leanprover/lean,leanprover/lean,leanprover/lean,leodemoura/lean,leodemoura/lean,Kha/lean,leodemoura/lean,leodemoura/lean,dselsam/lean,leanprover-community/lean,digama0/lean,dselsam/lean,leodemoura/lean,dselsam/lean,leodemoura/lean,leanprover-community/lean