Cleanup: Add comment to render job flag

Wasn't exactly clear what this was doing before, and the logic using it
requires a bit of brain gymnastics to follow (with negations, nested
if's and continue's).

source/blender/windowmanager/WM_api.hh

@@ -1548,6 +1548,10 @@ void wmGetProjectionMatrix(float mat[4][4], const rcti *winrct);
 /* Threaded Jobs Manager. */
 enum eWM_JobFlag {
   WM_JOB_PRIORITY = (1 << 0),
+  /**
+   * Only one render job can run at a time, this tags them a such. New jobs with this flag will
+   * wait on previous ones to finish then.
+   */
   WM_JOB_EXCL_RENDER = (1 << 1),
   WM_JOB_PROGRESS = (1 << 2),
 };