doc: fix http.ClientRequest method descriptions

fix documentation for methods getHeader, setHeader and removeHeader
for http.ClientRequest class. The documentation said these functions
can be called but they're wasn't describe into the API description yet.

add parameters and general description for each methods.

PR-URL: nodejs/node#15163
Fixes: nodejs/node#15048
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>