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/email_sync/changes/sync_member_email_to_user.ex

@@ -10,6 +10,21 @@ defmodule Mv.EmailSync.Changes.SyncMemberEmailToUser do
   use Ash.Resource.Change
   alias Mv.EmailSync.{Helpers, Loader}
 
+  @doc """
+  Implements the email synchronization from Member to User.
+
+  This function is called automatically by Ash when the configured trigger
+  conditions are met (see `@moduledoc` for trigger details).
+
+  ## Parameters
+  - `changeset` - The Ash changeset being processed
+  - `_opts` - Options passed to the change (unused)
+  - `context` - Ash context map containing metadata (e.g., `:syncing_email` flag)
+
+  ## Returns
+  Modified changeset with email synchronization applied, or original changeset
+  if recursion detected.
+  """
   @impl true
   def change(changeset, _opts, context) do
     # Only recursion protection needed - trigger logic is in `where` clauses

lib/mv/email_sync/changes/sync_user_email_to_member.ex

@@ -12,6 +12,21 @@ defmodule Mv.EmailSync.Changes.SyncUserEmailToMember do
   use Ash.Resource.Change
   alias Mv.EmailSync.{Helpers, Loader}
 
+  @doc """
+  Implements the email synchronization from User to Member.
+
+  This function is called automatically by Ash when the configured trigger
+  conditions are met (see `@moduledoc` for trigger details).
+
+  ## Parameters
+  - `changeset` - The Ash changeset being processed
+  - `_opts` - Options passed to the change (unused)
+  - `context` - Ash context map containing metadata (e.g., `:syncing_email` flag)
+
+  ## Returns
+  Modified changeset with email synchronization applied, or original changeset
+  if recursion detected.
+  """
   @impl true
   def change(changeset, _opts, context) do
     # Only recursion protection needed - trigger logic is in `where` clauses