Documents the ImageView.ScaleType attributes

Although the ImageView.ScaleType enum is documented, that
documentation was not reproduced in the xml attributes.  Copying the
documentation across should fill the previously empty 'Description'
column in
http://developer.android.com/reference/android/R.attr.html#scaleType
and
http://developer.android.com/reference/android/widget/ImageView.html#attr_android:scaleType

Also adds javadocs to ImageView.setImageMatrix(Matrix) because the
ScaleType documentation refers to it and it was also empty.

Change-Id: I7768549970eab2ebfe38cc4677a58dc91e104588