doc: issue tracking: explain the term issue

Transforms the loose enumeration of issues that was to far below
in the text into a more verbose paragraph at the top. The new text
also mentions the possibility to propose new types of issues to
track on the development mailing list.

Signed-off-by: Alexander Strasser <eclipse7@gmx.net>

doc/issue_tracker.txt

@@ -9,6 +9,13 @@ Overview:
 FFmpeg uses Trac for tracking issues, new issues and changes to
 existing issues can be done through a web interface.
 
+Issues can be different kinds of things we want to keep track of
+but that do not belong into the source tree itself. This includes
+bug reports, patches, feature requests and license violations. We
+might add more items to this list in the future, so feel free to
+propose a new `type of issue' on the ffmpeg-devel mailing list if
+you feel it is worth tracking.
+
 It is possible to subscribe to individual issues by adding yourself to the
 Cc list or to subscribe to the ffmpeg-trac mailing list which receives
 a mail for every change to every issue.
@@ -19,8 +26,6 @@ http(s)://ffmpeg.org/mailman/listinfo/ffmpeg-trac
 The URL of the webinterface of the tracker is:
 http(s)://ffmpeg.org/trac/ffmpeg
 
-NOTE: issue = (bug report || patch || feature request || license violation)
-
 Type:
 -----
 bug / defect