Missing typing for collection.update({},{},{multi: true})

monk current index.d.ts does not have definition for options ```{multi: true}``` for ```collection.update```. i think it great to have it since monk already support it.

Open index.d.ts and inspect the collection.update declaration and its existing option types. Confirm how the multi option is represented in Monk's supported update API, then add the corresponding typing so collection.update({}, {}, {multi: true}) is accepted and the declaration remains consistent with nearby options.

Written by the indexing model from the issue text.