update documentation to reflect DEBUG_LOG for release builds

CONTRIBUTING.md

@@ -48,9 +48,8 @@ environment was used and so on. Please tell us at least the following things:
 Remember, the easier it is for us to reproduce the bug, the earlier it will be
 corrected!
 
-In addition, you can compile restic with debug support by running
-`go run build.go -tags debug` and instructing it to create a debug
-log by setting the environment variable `DEBUG_LOG` to a file, e.g. like this:
+In addition, you can instruct restic to create a debug log by setting the
+environment variable `DEBUG_LOG` to a file, e.g. like this:
 
     $ export DEBUG_LOG=/tmp/restic-debug.log
     $ restic backup ~/work

doc/090_participating.rst

@@ -14,18 +14,12 @@
 Participating
 #############
 
-*********
-Debugging
-*********
+**********
+Debug Logs
+**********
 
-The program can be built with debug support like this:
-
-.. code-block:: console
-
-    $ go run build.go -tags debug
-
-Afterwards, extensive debug messages are written to the file in
-environment variable ``DEBUG_LOG``, e.g.:
+Set the environment variable ``DEBUG_LOG`` to let restic write extensive debug
+messages to the specified filed, e.g.:
 
 .. code-block:: console
 
@@ -66,6 +60,21 @@ statements originating in functions that match the pattern ``*unlock*``
     $ DEBUG_FUNCS=*unlock* restic check
 
 
+*********
+Debugging
+*********
+
+The program can be built with debug support like this:
+
+.. code-block:: console
+
+    $ go run build.go -tags debug
+
+This will make the ``restic debug <subcommand>`` available which can be used to
+inspect internal data structures. In addition, this enables profiling support
+which can help with investigation performance and memory usage issues.
+
+
 ************
 Contributing
 ************