Missing Documentation for ESP8266HTTPClient

### Basic Infos

- [x] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [x] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [x] I have tested that the issue is present in current master branch (aka latest git).
- [x] I have searched the issue tracker for a similar issue.
- [x] If there is a stack dump, I have decoded it.
- [x] I have filled out all fields below.

#### Platform
Does not apply, since issue is missing documentation.

### Settings in IDE
Does not apply, since issue is missing documentation.

### Problem Description
In the repository is the library `ESP8266HTTPClient`. There are some examples on the internet on how to use it, but there is no documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest). So using the library requires reading the source code (but at least for me not everything became clear from that).