[core] address review comments for message deletion explanation

...ject/briar/android/conversation/ConversationActivity.java

...arproject/briar/api/conversation/ConversationManager.java

...g/briarproject/briar/api/conversation/DeletionResult.java

...arproject/briar/introduction/IntroductionManagerImpl.java

...r/privategroup/invitation/GroupInvitationManagerImpl.java

...va/org/briarproject/briar/sharing/SharingManagerImpl.java

...oject/briar/introduction/IntroductionIntegrationTest.java

...ject/briar/messaging/MessagingManagerIntegrationTest.java

...ivategroup/invitation/GroupInvitationIntegrationTest.java

...iarproject/briar/sharing/ForumSharingIntegrationTest.java