bitbake: prserv/db: Document history modes

I keep having to dig into the archives to remember this information.
Add it as a comment to the file instead.

(Bitbake rev: 21dce82056887d8d28edde61b1b82f78bdf7613c)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/prserv/db.py

@@ -15,6 +15,16 @@ sqlversion = sqlite3.sqlite_version_info
 if sqlversion[0] < 3 or (sqlversion[0] == 3 and sqlversion[1] < 3):
     raise Exception("sqlite3 version 3.3.0 or later is required.")
 
+#
+# "No History" mode - for a given query tuple (version, pkgarch, checksum),
+# the returned value will be the largest among all the values of the same
+# (version, pkgarch). This means the PR value returned can NOT be decremented.
+#
+# "History" mode - Return a new higher value for previously unseen query
+# tuple (version, pkgarch, checksum), otherwise return historical value.
+# Value can decrement if returning to a previous build.
+#
+
 class PRTable(object):
     def __init__(self, conn, table, nohist):
         self.conn = conn