Deprecate server.enhanceMiddleware option

Summary:
Marks the `server.enhanceMiddleware` config option as deprecated.

This follows a review of this option's design:
- **Usage pitfall**: Extending Metro's builtin middleware by using the `metroMiddleware` arg is unclear/unergonomic (though now documented in D46228146).
- **Wrong location for this capability**: More importantly, the need to attach custom `connect` middleware at the level of Metro's config feels incorrect. This is an option for dev servers which use Metro, and can instead be configured — with more precision — when these consumers access Metro via its `runMetro` API.

Changelog: **[Deprecation]** Deprecate `server.enhanceMiddleware` option

Reviewed By: motiz88

Differential Revision: D46356577

fbshipit-source-id: 5486546390c0f750767353d8ac7408be624261e1

docs/Configuration.md

@@ -605,7 +605,7 @@ Type: `boolean`
 
 Whether we should enable CMD+R hotkey for refreshing the bundle.
 
-#### `enhanceMiddleware`
+#### `enhanceMiddleware` <div class="label deprecated">Deprecated</div>
 
 Type: `(Middleware, MetroServer) => Middleware`
 

packages/metro-config/src/configTypes.flow.js

@@ -164,6 +164,7 @@ type MetalConfigT = {
 };
 
 type ServerConfigT = {
+  /** @deprecated */
   enhanceMiddleware: (Middleware, MetroServer) => Middleware | Server,
   port: number,
   rewriteRequestUrl: string => string,

packages/metro-config/types/configTypes.d.ts

@@ -159,6 +159,7 @@ export interface MetalConfigT {
 }
 
 export interface ServerConfigT {
+  /** @deprecated */
   enhanceMiddleware: (
     metroMiddleware: Middleware,
     metroServer: MetroServer,