Extend documentation of core.askpass and GIT_ASKPASS.

Signed-off-by: Knut Franke <k.franke@science-computing.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -453,8 +453,11 @@ core.excludesfile::
 core.askpass::
 	Some commands (e.g. svn and http interfaces) that interactively
 	ask for a password can be told to use an external program given
-	via the value of this variable when it is set, and the
-	environment variable `GIT_ASKPASS` is not set.
+	via the value of this variable. Can be overridden by the 'GIT_ASKPASS'
+	environment variable. If not set, fall back to the value of the
+	'SSH_ASKPASS' environment variable or, failing that, a simple password
+	prompt. The external program shall be given a suitable prompt as
+	command line argument and write the password on its STDOUT.
 
 core.editor::
 	Commands such as `commit` and `tag` that lets you edit

Documentation/git.txt

@@ -634,6 +634,13 @@ Usually it is easier to configure any desired options through your
 personal `.ssh/config` file.  Please consult your ssh documentation
 for further details.
 
+'GIT_ASKPASS'::
+	If this environment variable is set, then git commands which need to
+	acquire passwords or passphrases (e.g. for HTTP or IMAP authentication)
+	will call this program with a suitable prompt as command line argument
+	and read the password from its STDOUT. See also the 'core.askpass'
+	option in linkgit:git-config[1].
+
 'GIT_FLUSH'::
 	If this environment variable is set to "1", then commands such
 	as 'git blame' (in incremental mode), 'git rev-list', 'git log',