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

Signed-off-by: Masatake YAMATO <[email protected]>

Author: masatake

docs/contributions.rst

@@ -438,12 +438,27 @@ Title of commit log and pull request
   Tmain
     Changes for the test cases under ``Tmain/`` directory
 
+  units
+    Changes for the test harness (misc/units.py) itself
+
   docs(web)
     Changes for the ``docs/*.rst``
 
   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