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/validations/email_not_used_by_other_member.ex

@@ -9,6 +9,22 @@ defmodule Mv.Accounts.User.Validations.EmailNotUsedByOtherMember do
   """
   use Ash.Resource.Validation
 
+  @doc """
+  Validates email uniqueness across linked User-Member pairs.
+
+  This validation ensures that when a user is linked to a member, their email
+  does not conflict with another member's email. It only runs when necessary
+  to avoid blocking valid operations (see `@moduledoc` for trigger conditions).
+
+  ## Parameters
+  - `changeset` - The Ash changeset being validated
+  - `_opts` - Options passed to the validation (unused)
+  - `_context` - Ash context map (unused)
+
+  ## Returns
+  - `:ok` if validation passes or should be skipped
+  - `{:error, field: :email, message: ..., value: ...}` if validation fails
+  """
   @impl true
   def validate(changeset, _opts, _context) do
     email_changing? = Ash.Changeset.changing_attribute?(changeset, :email)