[Docs] Fix up docs around key mapping (#46)

### Motivation

The docs had an incorrect name for `KeyMappingProvider`, plus they
didn't really show how to use the convenience methods.

### Modifications

Fixed the name and added examples of the convenience method.

### Result

Improved docs.

### Test Plan

Previewed locally.

Author: czechboy0

Sources/Configuration/Providers/Wrappers/KeyMappingProvider.swift

@@ -28,7 +28,7 @@
 ///
 /// ## Example
 ///
-/// Use `MappedKeyProvider` when you want to map keys for specific providers in a multi-provider
+/// Use `KeyMappingProvider` when you want to map keys for specific providers in a multi-provider
 /// setup:
 ///
 /// ```swift
@@ -49,6 +49,18 @@
 /// // This reads from "MYAPP_PROD_DATABASE_HOST" env var or "database.host" in JSON
 /// let host = config.string(forKey: "database.host", default: "localhost")
 /// ```
+///
+/// ## Convenience method
+///
+/// You can also use the ``ConfigProvider/prefixKeys(with:)`` convenience method on
+/// configuration provider types to wrap one in a ``KeyMappingProvider``:
+///
+/// ```swift
+/// let envProvider = EnvironmentVariablesProvider()
+/// let keyMappedEnvProvider = envProvider.mapKeys { key in
+///     key.prepending(["myapp", "prod"])
+/// }
+/// ```
 public struct KeyMappingProvider<Upstream: ConfigProvider>: Sendable {
     /// The mapping function applied to each key before a lookup.
     private let mapKey: @Sendable (AbsoluteConfigKey) -> AbsoluteConfigKey