docs: document credential.helper allowed values

gitcredentials(7) already mentions several possible invocations that one
can use as the value for credential.helper.  However, many people are
not aware that there are other options than a simple credential helper
name, so let's place some explanatory text in the documentation for
credential.helper as well.

We still refer the user to gitcredential(7) for additional explanations
and helpful examples.

Signed-off-by: brian m. carlson <[email protected]>
Acked-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

Documentation/config/credential.txt

@@ -1,9 +1,13 @@
 credential.helper::
 	Specify an external helper to be called when a username or
 	password credential is needed; the helper may consult external
-	storage to avoid prompting the user for the credentials. Note
-	that multiple helpers may be defined. See linkgit:gitcredentials[7]
-	for details.
+	storage to avoid prompting the user for the credentials. This is
+	normally the name of a credential helper with possible
+	arguments, but may also be an absolute path with arguments or, if
+	preceded by `!`, shell commands.
++
+Note that multiple helpers may be defined. See linkgit:gitcredentials[7]
+for details and examples.
 
 credential.useHttpPath::
 	When acquiring credentials, consider the "path" component of an http