completion: add some documentation
completion: add some documentation

fxfactorial/merlin,rgrinberg/merlin,rgrinberg/merlin,dra27/merlin,haselwarter/merlin,dra27/merlin,neojski/merlin,fxfactorial/merlin,chambart/merlin,rgrinberg/merlin,chambart/merlin,ocaml/merlin,chriskonstad/merlin,fxfactorial/merlin,haselwarter/merlin,the-lambda-church/merlin,chriskonstad/merlin,dra27/merlin,chambart/merlin,ocaml/merlin,chambart/merlin,haselwarter/merlin,the-lambda-church/merlin,neojski/merlin,haselwarter/merlin,neojski/merlin,dra27/merlin,ocaml/merlin,neojski/merlin,fxfactorial/merlin,ocaml/merlin,the-lambda-church/merlin,rgrinberg/merlin