Buck: Document how to build the documentation

Change-Id: I55a8e1ddd4eae34e2f1da3231d0c9c8f8e75bc79

Documentation/dev-buck.txt

diff --git a/Documentation/dev-buck.txt b/Documentation/dev-buck.txt
index 8fedd8a..12b0f96 100644
--- a/Documentation/dev-buck.txt
+++ b/Documentation/dev-buck.txt
@@ -177,6 +177,26 @@
 Note that when building an individual plugin, the `core.zip` package
 is not regenerated.
 
+
+[[documentation]]
+Documentation
+~~~~~~~~~~~~~
+
+To build the documentation:
+
+----
+  buck build docs
+----
+
+The generated html files will be placed in:
+
+----
+  buck-out/gen/Documentation
+----
+
+The html files will also be bundled into `html.zip` in the same location.
+
+
 [[release]]
 Gerrit Release WAR File
 ~~~~~~~~~~~~~~~~~~~~~~~