trace2: update docs to describe system/global config settings

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/trace2-target-values.txt b/Documentation/trace2-target-values.txt
new file mode 100644
index 0000000..27d3c64
--- /dev/null
+++ b/Documentation/trace2-target-values.txt
@@ -0,0 +1,10 @@
+--
+* `0` or `false` - Disables the target.
+* `1` or `true` - Writes to `STDERR`.
+* `[2-9]` - Writes to the already opened file descriptor.
+* `<absolute-pathname>` - Writes to the file in append mode.
+* `af_unix:[<socket_type>:]<absolute-pathname>` - Write to a
+Unix DomainSocket (on platforms that support them).  Socket
+type can be either `stream` or `dgram`; if omitted Git will
+try both.
+--