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: #15163
Fixes: #15048
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/http.md

@@ -286,9 +286,9 @@ added: v0.1.17
 
 This object is created internally and returned from [`http.request()`][].  It
 represents an _in-progress_ request whose header has already been queued.  The
-header is still mutable using the `setHeader(name, value)`, `getHeader(name)`,
-`removeHeader(name)` API.  The actual header will be sent along with the first
-data chunk or when calling [`request.end()`][].
+header is still mutable using the [`setHeader(name, value)`][],
+ [`getHeader(name)`][], [`removeHeader(name)`][] API.  The actual header will
+be sent along with the first data chunk or when calling [`request.end()`][].
 
 To get the response, add a listener for [`'response'`][] to the request object.
 [`'response'`][] will be emitted from the request object when the response
@@ -552,6 +552,58 @@ That's usually desired (it saves a TCP round-trip), but not when the first
 data is not sent until possibly much later.  `request.flushHeaders()` bypasses
 the optimization and kickstarts the request.
 
+### request.getHeader(name)
+<!-- YAML
+added: v1.6.0
+-->
+
+* `name` {string}
+* Returns: {string}
+
+Reads out a header on the request. Note that the name is case insensitive.
+
+Example:
+```js
+const contentType = request.getHeader('Content-Type');
+```
+
+### request.removeHeader(name)
+<!-- YAML
+added: v1.6.0
+-->
+
+* `name` {string}
+
+Removes a header that's already defined into headers object.
+
+Example:
+```js
+request.removeHeader('Content-Type');
+```
+
+### request.setHeader(name, value)
+<!-- YAML
+added: v1.6.0
+-->
+
+* `name` {string}
+* `value` {string}
+
+Sets a single header value for headers object. If this header already exists in
+the to-be-sent headers, its value will be replaced. Use an array of strings
+here to send multiple headers with the same name.
+
+Example:
+```js
+request.setHeader('Content-Type', 'application/json');
+```
+
+or
+
+```js
+request.setHeader('Set-Cookie', ['type=ninja', 'language=javascript']);
+```
+
 ### request.setNoDelay([noDelay])
 <!-- YAML
 added: v0.5.9
@@ -1904,6 +1956,7 @@ const req = http.request(options, (res) => {
 [`agent.createConnection()`]: #http_agent_createconnection_options_callback
 [`agent.getName()`]: #http_agent_getname_options
 [`destroy()`]: #http_agent_destroy
+[`getHeader(name)`]: #requestgetheadername
 [`http.Agent`]: #http_class_http_agent
 [`http.ClientRequest`]: #http_class_http_clientrequest
 [`http.IncomingMessage`]: #http_class_http_incomingmessage
@@ -1918,6 +1971,7 @@ const req = http.request(options, (res) => {
 [`net.Server`]: net.html#net_class_net_server
 [`net.Socket`]: net.html#net_class_net_socket
 [`net.createConnection()`]: net.html#net_net_createconnection_options_connectlistener
+[`removeHeader(name)`]: #requestremoveheadername
 [`request.end()`]: #http_request_end_data_encoding_callback
 [`request.setTimeout()`]: #http_request_settimeout_timeout_callback
 [`request.socket`]: #http_request_socket
@@ -1931,6 +1985,7 @@ const req = http.request(options, (res) => {
 [`response.writeContinue()`]: #http_response_writecontinue
 [`response.writeHead()`]: #http_response_writehead_statuscode_statusmessage_headers
 [`server.timeout`]: #http_server_timeout
+[`setHeader(name, value)`]: #requestsetheadername-value
 [`socket.setKeepAlive()`]: net.html#net_socket_setkeepalive_enable_initialdelay
 [`socket.setNoDelay()`]: net.html#net_socket_setnodelay_nodelay
 [`socket.setTimeout()`]: net.html#net_socket_settimeout_timeout_callback