git-svn: document svn.authorsProg in config

This has always been supported since we read config variables
based on the command-line option parser.  Document it explicitly
since users usually want to maintain the same program across
invocations.

Signed-off-by: Eric Wong <e@80x24.org>

Documentation/git-svn.txt

@@ -625,6 +625,9 @@ config key: svn.authorsfile
 	with the committer name as the first argument.  The program is
 	expected to return a single line of the form "Name <email>",
 	which will be treated as if included in the authors file.
++
+[verse]
+config key: svn.authorsProg
 
 -q::
 --quiet::