Add documentation about support for binding methods in companion obje…

…cts.

Issue #4287.

RELNOTES=n/a
PiperOrigin-RevId: 633637694

dev-guide/index.md

@@ -142,8 +142,8 @@ needs the module at compile time, and can avoid class loading the module at
 runtime.
 {: .c-callouts__note }
 
-Finally, all `@Provides` methods must belong to a module. These are just classes
-that have an [`@Module`][Module] annotation.
+Finally, all `@Provides` and `@Binds` methods must belong to a module. These are
+just classes that have an [`@Module`][Module] annotation.
 
 ```java
 @Module
@@ -152,6 +152,20 @@ interface HeaterModule {
 }
 ```
 
+Note that in Kotlin, `@Provides` methods can also be declared in the companion
+object of an `@Module` class.
+
+```kotlin
+@Module
+interface HeaterModule {
+  @Binds fun bindHeater(impl: ElectricHeater): Heater
+
+  companion object {
+    @Provides fun provideElectricHeater() = ElectricHeater()
+  }
+}
+```
+
 By convention, `@Provides` methods are named with a `provide` prefix, `@Binds`
 methods are named with `bind` prefix and module classes are named with a
 `Module` suffix.