Mark http/2 support as stable (#3893)

Signed-off-by: Matteo Collina <hello@matteocollina.com>

docs/docs/api/Client.md

@@ -17,8 +17,6 @@ Returns: `Client`
 
 ### Parameter: `ClientOptions`
 
-> ⚠️ Warning: The `H2` support is experimental.
-
 * **bodyTimeout** `number | null` (optional) - Default: `300e3` - The timeout after which a request will time out, in milliseconds. Monitors time between receiving body data. Use `0` to disable it entirely. Defaults to 300 seconds. Please note the `timeout` will be reset if you keep writing data to the socket everytime.
 * **headersTimeout** `number | null` (optional) - Default: `300e3` - The amount of time, in milliseconds, the parser will wait to receive the complete HTTP headers while not sending the request. Defaults to 300 seconds.
 * **keepAliveMaxTimeout** `number | null` (optional) - Default: `600e3` - The maximum allowed `keepAliveTimeout`, in milliseconds, when overridden by *keep-alive* hints from the server. Defaults to 10 minutes.
@@ -34,6 +32,17 @@ Returns: `Client`
 * **allowH2**: `boolean` - Default: `false`. Enables support for H2 if the server has assigned bigger priority to it through ALPN negotiation.
 * **maxConcurrentStreams**: `number` - Default: `100`. Dictates the maximum number of concurrent streams for a single H2 session. It can be overridden by a SETTINGS remote frame.
 
+> **Notes about HTTP/2**
+> - It only works under TLS connections. h2c is not supported.
+> - The server must support HTTP/2 and choose it as the protocol during the ALPN negotiation.
+>   - The server must not have a bigger priority for HTTP/1.1 than HTTP/2.
+> - Pseudo headers are automatically attached to the request. If you try to set them, they will be overwritten.
+>   - The `:path` header is automatically set to the request path.
+>   - The `:method` header is automatically set to the request method.
+>   - The `:scheme` header is automatically set to the request scheme.
+>   - The `:authority` header is automatically set to the request `host[:port]`.
+> - `PUSH` frames are yet not supported.
+
 #### Parameter: `ConnectOptions`
 
 Every Tls option, see [here](https://nodejs.org/api/tls.html#tls_tls_connect_options_callback).

lib/dispatcher/client-h2.js

@@ -35,9 +35,6 @@ const kOpenStreams = Symbol('open streams')
 
 let extractBody
 
-// Experimental
-let h2ExperimentalWarned = false
-
 /** @type {import('http2')} */
 let http2
 try {
@@ -82,13 +79,6 @@ function parseH2Headers (headers) {
 async function connectH2 (client, socket) {
   client[kSocket] = socket
 
-  if (!h2ExperimentalWarned) {
-    h2ExperimentalWarned = true
-    process.emitWarning('H2 support is experimental, expect them to change at any time.', {
-      code: 'UNDICI-H2'
-    })
-  }
-
   const session = http2.connect(client[kUrl], {
     createConnection: () => socket,
     peerMaxConcurrentStreams: client[kMaxConcurrentStreams],