hwc: update blank documentation to specify that blank is synchronous

Change the documentation of the blank function in the hw composer to
specify that the screen state transition must be complete when the
function returns.

Change-Id: Ibd0a9cdef13991c36cabada5fc0e4f7bb8fa7af9

include/hardware/hwcomposer.h

@@ -85,6 +85,8 @@ typedef struct hwc_methods_1 {
      *
      * Turns the screen off when blank is nonzero, on when blank is zero.
      * Multiple sequential calls with the same blank value must be supported.
+     * The screen state transition must be be complete when the function
+     * returns.
      *
      * returns 0 on success, negative on error.
      */