godoc(term): unclear what `fd.utptr` is

We should definitely include some common file descriptors for terminal apps, given that's the most likely usage. Maybe even link to other kinds of file descriptors. Fwiw I'm using with the `term.GetSize()` func

Start with the godoc for fd.utptr and the term.GetSize entry point to determine which terminal file descriptors the documentation currently explains. Done means the godoc clearly identifies common terminal descriptors and links to relevant other file-descriptor types.

Written by the indexing model from the issue text.