Update ImagenPersonFilterLevel refdocs to match the iOS SDK (#6995)

This PR updates the ImagenPersonFilter refdocs to match the [iOS
SDK](https://github.com/firebase/firebase-ios-sdk/blob/4f6c342424df416d78dfc12d08c97769fd4e1152/FirebaseAI/Sources/Types/Public/Imagen/ImagenPersonFilterLevel.swift#L33-L45),
including the information about the [Person and face generation
allowlist](https://cloud.google.com/vertex-ai/generative-ai/docs/image/responsible-ai-imagen#person-face-gen).

Author: thatfiredev

firebase-ai/src/main/kotlin/com/google/firebase/ai/type/ImagenPersonFilterLevel.kt

@@ -20,12 +20,30 @@ package com.google.firebase.ai.type
 @PublicPreviewAPI
 public class ImagenPersonFilterLevel private constructor(internal val internalVal: String) {
   public companion object {
-    /** No filters applied. */
+    /**
+     * Allow generation of images containing people of all ages.
+     *
+     * > Important: Generation of images containing people or faces may require your use case to be
+     * reviewed and approved by Cloud support; see the
+     * [Responsible AI and usage
+     * guidelines](https://cloud.google.com/vertex-ai/generative-ai/docs/image/responsible-ai-imagen#person-face-gen)
+     * for more details.
+     */
     @JvmField public val ALLOW_ALL: ImagenPersonFilterLevel = ImagenPersonFilterLevel("allow_all")
-    /** Filters out any images containing depictions of children. */
+    /**
+     * Allow generation of images containing adults only; images of children are filtered out.
+     *
+     * > Important: Generation of images containing people or faces may require your use case to be
+     * reviewed and approved by Cloud support; see the
+     * [Responsible AI and usage
+     * guidelines](https://cloud.google.com/vertex-ai/generative-ai/docs/image/responsible-ai-imagen#person-face-gen)
+     * for more details.
+     */
     @JvmField
     public val ALLOW_ADULT: ImagenPersonFilterLevel = ImagenPersonFilterLevel("allow_adult")
-    /** Filters out any images containing depictions of people. */
+    /**
+     * Disallow generation of images containing people or faces; images of people are filtered out.
+     */
     @JvmField public val BLOCK_ALL: ImagenPersonFilterLevel = ImagenPersonFilterLevel("dont_allow")
   }
 }