fix(model): disallow updateMany(update) and fix TypeScript types re: updateMany()

[vkarpov15]

Fix #15190

Summary

#15190 points out that Model.updateMany(obj) updates all documents using obj, which is risky, especially given that our TypeScript types as written imply that update parameter is optional, meaning updateMany(obj) treats obj as filter rather than update, but the runtime behavior is different.

This PR specifically blocks Model.updateMany(obj, null), while allowing Model.updateOne(obj) and Query.prototype.updateMany(obj, null).

Examples

[hasezoey]

LGTM, though i think this will require changes for some users.

[vkarpov15]

That is possible, however hopefully unlikely. The updateMany(doc) code path isn't in our current docs or tests. And this doesn't affect the chaining code path, just top-level updateMany().