Improving documentation for built-in type declarations

Looking at for instance lib.dom.d.ts, what I notice is that the documentation isn't what it could be. Is there any policy w.r.t. improving on this? I think the benefits of better docs are quite obvious, for instance adding examples or stating browser support for certain features.

However I wonder if:

• Since the file is already quite large, and adding documentation could blow it up like 100x, would this cause any performance issues (in editors)? If so, is there a way around it?
• The best documentation on these things (that I know of) are on MDN. A nice start could be to link/auto add some docs from there, if the licensing allows it (https://wiki.developer.mozilla.org/en-US/docs/MDN/About#Copyrights_and_licenses). I think it does at least in part.

Start by reviewing lib.dom.d.ts and the issue's questions about documentation size, editor performance, and MDN licensing. Compare the existing declarations with the referenced MDN material. This issue is complete only after the project agrees on a documentation policy and a concrete, accepted approach.