Update documentation that refers to removed APIs

[paulb777]

For example, activateFetched still has four references, which will make it challenging if developers need to replace it.

FirebaseRemoteConfig/Sources/Public/FirebaseRemoteConfig/FIRRemoteConfig.h:/// Remote Config Server until they are copied to the Active Config by calling activateFetched. When
FirebaseRemoteConfig/Sources/Public/FirebaseRemoteConfig/FIRRemoteConfig.h:/// they are copied to the Active Config by calling activateFetched. When you fetch values from the
FirebaseRemoteConfig/Sources/Public/FirebaseRemoteConfig/FIRRemoteConfig.h:/// Fetches Remote Config data with a callback. Call activateFetched to make fetched data available
FirebaseRemoteConfig/Sources/Public/FirebaseRemoteConfig/FIRRemoteConfig.h:/// Call activateFetched to make fetched data available to your app.