Add comment documenting makeRequest#headers type.

[dcr-stripe]

r? @richardm-stripe

Summary

Adds a comment documenting why HttpClient#makeRequest(headers) doesn't use a stricter type. Unfortunately this is implementation-specific and enforcing a stricter type could break some integrations.

As examples, we cannot just use {[key: string]: string} since:

1. Node actually supports {[key: string]: string | number | string[]}
2. Fetch actually also supports string[][] as well as a Headers object.