doc review: the Job ID is currently a random SHA1 string

It no longer attempts to encode any kind of information. Let's also
give examples on how the full and the short form of the SHA1 strings
are used.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

docs/source/ReferenceGuide.rst

@@ -11,15 +11,18 @@ This guide presents information on the Avocado basic design and its internals.
 Job ID
 ======
 
-The Job ID is a SHA1 string that has some information encoded:
+The Job ID is a random SHA1 string that uniquely identifies a given job.
 
-* Hostname
-* ISO timestamp
-* 64 bit integer
+The full form of the SHA1 string is used is most references to a job::
 
-The idea is to have a unique identifier that can be used for job data, for
-the purposes of joining on a single database results obtained by jobs run
-on different systems.
+  $ avocado run sleeptest
+  JOB ID     : 49ec339a6cca73397be21866453985f88713ac34
+  ...
+
+But a shorter version is also used at some places, such as in the job
+results location::
+
+  JOB LOG    : $HOME/avocado/job-results/job-2015-06-10T10.44-49ec339/job.log
 
 .. _test-types: