Documentation for default option is confusing

[ssokolow]

Currently, if I add a default = "8000" to my port: Option<u16> without removing the Option, it gives the following error message.

the trait `std::str::FromStr` is not implemented for `std::option::Option<u16>`

I found this extremely confusing because I didn't try compiling it before adding the default and the documentation doesn't mention that adding default will narrow the set of valid type signatures, and the Option<i32> example from the docs was also breaking.

(I went in circles double-checking these examples for 15 minutes and looking at the FromStr API docs before it occurred to me that maybe default altered the set of valid types.)

The documentation for default should be amended to mention this.