Address PR comments

Add more docs.
Rename PhotoPickerPresenter to MediaPickerPresenter.
Use a Character for the placeholder avatar rather than a string.

RiotSwiftUI/Modules/Onboarding/DisplayName/OnboardingDisplayNameModels.swift

@@ -19,7 +19,9 @@ import Foundation
 // MARK: View model
 
 enum OnboardingDisplayNameViewModelResult {
+    /// The user would like to save the entered display name.
     case save(String)
+    /// Move on to the next screen in the flow without setting a display name.
     case skip
 }
 
@@ -27,20 +29,27 @@ enum OnboardingDisplayNameViewModelResult {
 
 struct OnboardingDisplayNameViewState: BindableState {
     var bindings: OnboardingDisplayNameBindings
+    /// Any error that occurred during display name validation otherwise `nil`.
     var validationErrorMessage: String?
     
+    /// The string to be displayed in the text field's footer.
     var textFieldFooterMessage: String {
         validationErrorMessage ?? VectorL10n.onboardingDisplayNameHint
     }
 }
 
 struct OnboardingDisplayNameBindings {
+    /// The display name string entered by the user.
     var displayName: String
+    /// The currently displayed alert's info value otherwise `nil`.
     var alertInfo: AlertInfo<Int>?
 }
 
 enum OnboardingDisplayNameViewAction {
+    /// The display name needs validation.
     case validateDisplayName
+    /// The user would like to save the entered display name.
     case save
+    /// Move on to the next screen in the flow without setting a display name.
     case skip
 }