gstreamer: docs: document GST_DEBUG env var evaluation order

Fixes #716

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/923>

diff --git a/subprojects/gstreamer/docs/gst/running.md b/subprojects/gstreamer/docs/gst/running.md
index df5f4eb..e6b1e3b 100644 (file)
--- a/subprojects/gstreamer/docs/gst/running.md
+++ b/subprojects/gstreamer/docs/gst/running.md
@@ -131,6 +131,12 @@ hand.
 Since GStreamer 1.2 it is also possible to specify debug levels by name,
 e.g. `GST_DEBUG=*:WARNING,*audio*:LOG`
 
+The categories and log levels are evaluated from left to right, with later
+entries overriding any previously-set levels for categories, e.g.
+`GST_DEBUG=decodebin:LOG,*:INFO` will set the decodebin category back to
+INFO level whereas `GST_DEBUG=*:INFO,decodebin:LOG` will log everything at
+INFO level apart from decodebin which will be logged at LOG level.
+
 **`GST_DEBUG_NO_COLOR`.**
 
 Set this environment variable to any value ("1" typically) to switch