Document CAPTURE and CAPTURE_ALL.

wiki/src/contribute/release_process/test/usage.mdwn

@@ -72,6 +72,13 @@ can be use by concurrent test suite runs):
 Here's a list of all non-secret key-value pairs that can be supported
 by the local configuration file:
 
+* `CAPTURE`: Captures failed scenarios into videos stored in the
+  temporary directory (see `TMPDIR` below) using x264
+  encoding. Defaults to `false`.
+
+* `CAPTURE_ALL`: Keep videos for all scenarios, including those that
+  succeed (implies `CAPTURE`). Defaults to `false`.
+
 * `DEBUG`: Boolean value. If set to `true`, various debugging info
   will be printed. Defaults to `false`.