org-clock: Mention semimonth step in manual and add test

* doc/org-manual.org (The clock table): Mention new semimonth :step
value.
* testing/lisp/test-org-clock.el (test-org-clock/clocktable/step):
Test :step semimonth.

Add a NEWS entry as well.

doc/org-manual.org

@@ -6655,9 +6655,9 @@ be selected:
 
 - =:step= ::
 
-  Set to =day=, =week=, =month= or =year= to split the table into
-  chunks.  To use this, either =:block=, or =:tstart= and =:tend= are
-  required.
+  Set to =day=, =week=, =semimonth, =month= or =year= to split the
+  table into chunks.  To use this, either =:block=, or =:tstart= and
+ =:tend= are required.
 
 - =:stepskip0= ::
 

etc/ORG-NEWS

@@ -175,6 +175,8 @@ option ~org-display-remote-inline-images~.
 `k' option, but the user specifies a time of day, not a number of
 minutes.
 
+*** New step value =semimonth= accepted for clock tables
+
 *** Allow text rescaling in column view
 
 You can now use =C-x C-+= in column view: the columns face size will

testing/lisp/test-org-clock.el

@@ -1053,6 +1053,39 @@ CLOCK: [2014-04-03 Thu 08:00]--[2014-04-03 Thu 16:00] =>  8:00"
       (let ((system-time-locale "en_US"))
         (test-org-clock-clocktable-contents
 	    ":step month :mstart 4 :block 2014 :stepskip0 t")))))
+  ;; Test ":step semimonth".
+  (should
+   (string-match-p
+    "
+.*?\\[2014-03-01 .*
+.*
+.*
+|.*?| \\*8:00\\* |
+.*
+| Foo +| 8:00 +|
+
+.*?\\[2014-03-16 .*
+.*
+.*
+|.*?| \\*2:00\\* |
+.*
+| Foo +| 2:00 +|
+
+.*?\\[2014-04-01 .*
+.*
+.*
+|.*?| \\*7:00\\* |
+.*
+| Foo +| 7:00 +|
+"
+    (org-test-with-temp-text
+     "* Foo
+CLOCK: [2014-03-04 Tue 08:00]--[2014-03-04 Tue 16:00] =>  8:00
+CLOCK: [2014-03-24 Mon 08:00]--[2014-03-24 Mon 10:00] =>  2:00
+CLOCK: [2014-04-03 Thu 08:00]--[2014-04-03 Thu 15:00] =>  7:00"
+     (let ((system-time-locale "en_US"))
+       (test-org-clock-clocktable-contents
+	":step semimonth :block 2014 :stepskip0 t")))))
   ;; Test ":step year".
   (should
    (string-match-p