add documentation for firebase app

firebase-app/src/commonMain/kotlin/dev/gitlive/firebase/firebase.kt

@@ -12,13 +12,38 @@ import kotlin.jvm.JvmName
 /**
  * Single access point to all firebase sdks from Kotlin.
  *
- * <p>Acts as a target for extension methods provided by sdks.
+ * Acts as a target for extension methods provided by sdks.
  */
 object Firebase
 
+/**
+ * The entry point of Firebase SDKs. It holds common configuration and state for Firebase APIs. Most
+ * applications don't need to directly interact with FirebaseApp.
+ *
+ * For a vast majority of apps, FirebaseInitProvider will handle the initialization of
+ * Firebase for the default project that it's configured to work with, via the data contained in the
+ * app's `google-services.json` file. This `ContentProvider`
+ * is merged into the app's manifest by default when building with Gradle,
+ * and it runs automatically at app launch. No additional lines of code are needed in this
+ * case.
+ *
+ * Any `FirebaseApp` initialization must occur only in the main process of the app.
+ * Use of Firebase in processes other than the main process is not supported and will likely cause
+ * problems related to resource contention.
+ */
 expect class FirebaseApp {
+    /** Returns the unique name of this app. */
     val name: String
+
+    /** Returns the specified [FirebaseOptions]. */
     val options: FirebaseOptions
+
+    /**
+     * Deletes the [FirebaseApp] and all its data. All calls to this [FirebaseApp]
+     * instance will throw once it has been called.
+     *
+     * A no-op if delete was called before.
+     */
     suspend fun delete()
 }
 
@@ -44,22 +69,58 @@ expect fun Firebase.initialize(context: Any? = null, options: FirebaseOptions, n
 val Firebase.options: FirebaseOptions
     get() = Firebase.app.options
 
+/** Configurable Firebase options. */
 data class FirebaseOptions(
+    /** The Google App ID that is used to uniquely identify an instance of an app. */
     val applicationId: String,
+
+    /**
+     * API key used for authenticating requests from your app, e.g.
+     * AIzaSyDdVgKwhZl0sTTTLZ7iTmt1r3N2cJLnaDk, used to identify your app to Google servers.
+     */
     val apiKey: String,
+
+    /** The database root URL, e.g. http://abc-xyz-123.firebaseio.com. */
     val databaseUrl: String? = null,
+
+    /**
+     * The tracking ID for Google Analytics, e.g. UA-12345678-1, used to configure Google Analytics.
+     */
     val gaTrackingId: String? = null,
+
+    /** The Google Cloud Storage bucket name, e.g. abc-xyz-123.storage.firebase.com. */
     val storageBucket: String? = null,
+
+    /** The Google Cloud project ID, e.g. my-project-1234 */
     val projectId: String? = null,
+
+    /**
+     * The Project Number from the Google Developer's console, for example 012345678901, used to
+     * configure Google Cloud Messaging.
+     */
     val gcmSenderId: String? = null,
+
+    /** The auth domain. */
     val authDomain: String? = null
 )
 
+/**
+ * Exception that gets thrown when an operation on Firebase fails.
+ */
 expect open class FirebaseException : Exception
 
+/**
+ * Exception that gets thrown when an operation on Firebase fails.
+ */
 expect class FirebaseNetworkException : FirebaseException
 
+/**
+ * Exception that gets thrown when an operation on Firebase fails.
+ */
 expect open class FirebaseTooManyRequestsException : FirebaseException
 
+/**
+ * Exception that gets thrown when an operation on Firebase fails.
+ */
 expect open class FirebaseApiNotAvailableException : FirebaseException