docs(web): write more about prefixes used in commit headers

Signed-off-by: Masatake YAMATO <yamato@redhat.com>

docs/contributions.rst

@@ -435,15 +435,30 @@ Title of commit log and pull request
   Units:
     Changes for the test cases under ``Units/`` directory
 
-  Tmain
+  Tmain:
     Changes for the test cases under ``Tmain/`` directory
 
-  docs(web)
+  units:
+    Changes for the test harness (misc/units.py) itself
+
+  docs(web):
     Changes for the ``docs/*.rst``
 
-  docs(man)
+  docs(man):
     Changes for the ``man/*.rst``
 
+  dsl:
+    Changes operators that can be used in optscript command
+
+  operators:
+    Changes operators that can be used in optlib `{{` ... `}}` blocks
+
+  prelude:
+    Changes optlib procedures defined in main/CommonPrelude.ps
+
+  CXX (or Cxx):
+    Changes affecting all parsers defined in parsers/cxx
+
   See also the output of ``git log`` command.
 
 * Combine prefixes with a comma if a change modifies multiple parts of our source tree