docs: add @doc to public functions in EmailSync, Validations, and Senders

Document public API functions with @doc for better tooling support:
- EmailSync Changes: sync_user_email_to_member, sync_member_email_to_user
- Validations: email_not_used_by_other_member, email_not_used_by_other_user
- Senders: send_new_user_confirmation_email, send_password_reset_email

lib/mv/accounts/user/senders/send_new_user_confirmation_email.ex

@@ -10,6 +10,20 @@ defmodule Mv.Accounts.User.Senders.SendNewUserConfirmationEmail do
 
   alias Mv.Mailer
 
+  @doc """
+  Sends a confirmation email to a new user.
+
+  This function is called automatically by AshAuthentication when a new
+  user registers and needs to confirm their email address.
+
+  ## Parameters
+  - `user` - The user record who needs to confirm their email
+  - `token` - The confirmation token to include in the email link
+  - `_opts` - Additional options (unused)
+
+  ## Returns
+  The Swoosh.Email delivery result from `Mailer.deliver!/1`.
+  """
   @impl true
   def send(user, token, _) do
     new()

lib/mv/accounts/user/senders/send_password_reset_email.ex

@@ -10,6 +10,20 @@ defmodule Mv.Accounts.User.Senders.SendPasswordResetEmail do
 
   alias Mv.Mailer
 
+  @doc """
+  Sends a password reset email to a user.
+
+  This function is called automatically by AshAuthentication when a user
+  requests a password reset.
+
+  ## Parameters
+  - `user` - The user record requesting the password reset
+  - `token` - The password reset token to include in the email link
+  - `_opts` - Additional options (unused)
+
+  ## Returns
+  The Swoosh.Email delivery result from `Mailer.deliver!/1`.
+  """
   @impl true
   def send(user, token, _) do
     new()