Documentation for phone_number::Country is missing

[jbg]

At the moment it's necessary to look at the source to find e.g. the id() method on Country.

[meh]

I'm currently upgrading the parsers from nom 4 to nom 5, after I'm done with that I'll do a documentation pass.

Sorry about the annoyance 🐼

[rustonaut]

After cleaning thinks up I will go though the structure of the library this might be fixed then.