Enhance Per Diem API Documentation

The request and response documentation for the Per Diem API could provide greater detail. For instance, Endpoint 1: `https://api.gsa.gov/travel/perdiem/v2/rates/city/{city}/state/{ST}/year/{year}` could specify that `{ST}` requires a state abbreviation and does not support long form state names. The Expected Result claims two values in the response but in reality, it is much longer (see below).

```json
{
"request": null,
"errors": null,
"rates": [
{
"oconusInfo": null,
"rate": [
{
"months": {
"month": [
{
"value": 152,
"number": 1,
"short": "Jan",
"long": "January"
},
{
"value": 152,
"number": 2,
"short": "Feb",
"long": "February"
},
{
"value": 152,
"number": 3,
"short": "Mar",
"long": "March"
},
{
"value": 152,
"number": 4,
"short": "Apr",
"long": "April"
},
{
"value": 152,
"number": 5,
"short": "May",
"long": "May"
},
{
"value": 152,
"number": 6,
"short": "Jun",
"long": "June"
},
{
"value": 152,
"number": 7,
"short": "Jul",
"long": "July"
},
{
"value": 152,
"number": 8,
"short": "Aug",
"long": "August"
},
{
"value": 159,
"number": 9,
"short": "Sep",
"long": "September"
},
{
"value": 159,
"number": 10,
"short": "Oct",
"long": "October"
},
{
"value": 152,
"number": 11,
"short": "Nov",
"long": "November"
},
{
"value": 152,
"number": 12,
"short": "Dec",
"long": "December"
}
]
},
"meals": 66,
"zip": null,
"county": "Fulton / Dekalb",
"city": "Atlanta",
"standardRate": "false"
}
],
"state": "GA",
"year": 2019,
"isOconus": "false"
}
],
"version": null
}
```