graphics.mapper@4.0: Document previously-implicit accessRegion behavior.

Existing Android framework code (and transitively, CTS test) require
that an accessRegion of (0,0,0,0) is treated the same as an
accessRegion covering the entire buffer, when calling lock() or
lockYCbCr().

Document this so that there is no confusion about this going forward,
since this requirement pre-dates the HIDL HALs.

Bug: 119440345
Test: Builds, passes CTS

Change-Id: I5ff86539ee28a72dd972255ad405db357e62536d

graphics/mapper/4.0/IMapper.hal

@@ -206,6 +206,9 @@ interface IMapper {
      * outside of @p accessRegion is undefined, except that it must not cause
      * process termination.
      *
+     * An accessRegion of all-zeros means the entire buffer. That is, it is
+     * equivalent to '(0,0)-(buffer width, buffer height)'.
+     *
      * This function can lock both single-planar and multi-planar formats. The caller
      * should use get() to get information about the buffer they are locking.
      * get() can be used to get information about the planes, offsets, stride,