api-credentials.txt: mention credential.helper explicitly

The name of the configuration variable was mentioned only at the very
end of the explanation, in a place specific to a specific rule, hence it
was not very clear what the specification was about.

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

Documentation/technical/api-credentials.txt

@@ -179,8 +179,10 @@ credentials from and to long-term storage (where "long-term" is simply
 longer than a single git process; e.g., credentials may be stored
 in-memory for a few minutes, or indefinitely on disk).
 
-Each helper is specified by a single string. The string is transformed
-by git into a command to be executed using these rules:
+Each helper is specified by a single string in the configuration
+variable `credential.helper` (and others, see linkgit:../git-config[1]).
+The string is transformed by git into a command to be executed using
+these rules:
 
   1. If the helper string begins with "!", it is considered a shell
      snippet, and everything after the "!" becomes the command.