update doc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@34240 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/dbm/dbm.c

@@ -961,10 +961,12 @@ fdbm_reject(VALUE obj)
  * The exact library used depends on how Ruby was compiled. It could be any
  * of the following:
  *
+ * - The original ndbm is released in 4.3BSD.
+ *   It is based on dbm in Unix Version 7 but has different API.
  * - {Berkeley DB}[http://en.wikipedia.org/wiki/Berkeley_DB] versions
  *   1 thru 5, also known as BDB and Sleepycat DB, now owned by Oracle
  *   Corporation.
- * - ndbm, aka Berkeley DB 1.x, still found in FreeBSD and OpenBSD.
+ * - Berkeley DB 1.x, still found in FreeBSD and OpenBSD.
  * - {gdbm}[http://www.gnu.org/software/gdbm/], the GNU implementation of dbm.
  * - {qdbm}[http://fallabs.com/qdbm/index.html], another open source
  *   reimplementation of dbm.