Change phrasing to be clearer in the single_stream flag doc and add it to the docs.

doc/library/config.txt

@@ -485,6 +485,21 @@ import theano and print the config variable, as in:
         automatically to get more memory. But this can cause
         fragmentation, see note above.
 
+.. attribute:: config.gpuarray.single_stream
+
+    Boolean value
+
+    Default: ``True``
+
+    Control the stream mode of contexts.
+
+    If your computations are mostly lots of small elements, using
+    single-stream will avoid the synchronization overhead and usually
+    be faster.  For larger elements it does not make a difference yet.
+    In the future when true multi-stream is enabled in libgpuarray,
+    this may change.  If you want to make sure to have optimal
+    performance, check both options.
+
 
 .. attribute:: linker
 

theano/configdefaults.py

@@ -243,9 +243,14 @@ AddConfigVar('gpuarray.preallocate',
              in_c_key=False)
 
 AddConfigVar('gpuarray.single_stream',
-             """Switch between single stream mode or multi-stream
-             mode.  If your computation can't benefit from multiple
-             streams, single-stream is usually faster (by about 10%)
+             """
+             If your computations are mostly lots of small elements,
+             using single-stream will avoid the synchronization
+             overhead and usually be faster.  For larger elements it
+             does not make a difference yet.  In the future when true
+             multi-stream is enabled in libgpuarray, this may change.
+             If you want to make sure to have optimal performance,
+             check both options.
              """,
              BoolParam(True),
              in_c_key=False)