api-credential.txt: document that helpers field is filled-in automatically

It was unclear whether the field was to be specified by the user of the
API.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-credentials.txt

@@ -65,7 +65,10 @@ Data Structures
 The `helpers` member of the struct is a `string_list` of helpers.  Each
 string specifies an external helper which will be run, in order, to
 either acquire or store credentials. See the section on credential
-helpers below.
+helpers below. This list is filled-in by the API functions
+according to the corresponding configuration variables before
+consulting helpers, so there usually is no need for a caller to
+modify the helpers field at all.
 +
 This struct should always be initialized with `CREDENTIAL_INIT` or
 `credential_init`.