MINOR: add session windows doc to streams.html

Author: Damian Guy <damian.guy@gmail.com>

Reviewers: Michael G. Noll, Matthias J. Sax, Guozhang Wang

Closes #2547 from dguy/session-window-doc

docs/streams.html

@@ -539,6 +539,13 @@
         <li><b>Hopping time windows</b> are windows based on time intervals. They model fixed-sized, (possibly) overlapping windows. A hopping window is defined by two properties: the window's size and its advance interval (aka "hop"). The advance interval specifies by how much a window moves forward relative to the previous one. For example, you can configure a hopping window with a size 5 minutes and an advance interval of 1 minute. Since hopping windows can overlap a data record may belong to more than one such windows.</li>
         <li><b>Tumbling time windows</b> are a special case of hopping time windows and, like the latter, are windows based on time intervals. They model fixed-size, non-overlapping, gap-less windows. A tumbling window is defined by a single property: the window's size. A tumbling window is a hopping window whose window size is equal to its advance interval. Since tumbling windows never overlap, a data record will belong to one and only one window.</li>
         <li><b>Sliding windows</b> model a fixed-size window that slides continuously over the time axis; here, two data records are said to be included in the same window if the difference of their timestamps is within the window size. Thus, sliding windows are not aligned to the epoch, but on the data record timestamps. In Kafka Streams, sliding windows are used only for join operations, and can be specified through the <code>JoinWindows</code> class.</li>
+        <li><b>Session windows</b> are used to aggregate key-based events into sessions.
+            Sessions represent a period of activity separated by a defined gap of inactivity.
+            Any events processed that fall within the inactivity gap of any existing sessions are merged into the existing sessions.
+            If the event falls outside of the session gap, then a new session will be created.
+            Session windows are tracked independently across keys (e.g. windows of different keys typically have different start and end times) and their sizes vary (even windows for the same key typically have different sizes);
+            as such session windows can't be pre-computed and are instead derived from analyzing the timestamps of the data records.
+        </li>
         </ul>
 
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