TJ: Calc. xformed buf sizes based on dst. subsamp

With respect to tj3Transform(), this addresses an oversight from
bb1d540a80.

Note to self: A convenience function/method for computing the worst-case
transformed JPEG size for a particular transform would be nice.

turbojpeg.h

@@ -2065,13 +2065,15 @@ DLLEXPORT int tj3DecodeYUV8(tjhandle handle, const unsigned char *srcBuf,
  * you, or
  * -# pre-allocate the buffer to a "worst case" size determined by calling
  * #tj3JPEGBufSize() with the transformed or cropped width and height and the
- * level of subsampling used in the destination image.  Under normal
- * circumstances, this should ensure that the buffer never has to be
+ * level of subsampling used in the destination image (taking into account
+ * grayscale conversion and transposition of the width and height.)  Under
+ * normal circumstances, this should ensure that the buffer never has to be
  * re-allocated.  (Setting #TJPARAM_NOREALLOC guarantees that it won't be.)
  * Note, however, that there are some rare cases (such as transforming images
  * with a large amount of embedded Exif or ICC profile data) in which the
  * transformed JPEG image will be larger than the worst-case size, and
- * #TJPARAM_NOREALLOC cannot be used in those cases.
+ * #TJPARAM_NOREALLOC cannot be used in those cases unless the embedded data is
+ * discarded using #TJXOPT_COPYNONE.
  * .
  * If you choose option 1, then `dstSizes[i]` should be set to the size of your
  * pre-allocated buffer.  In any case, unless you have set #TJPARAM_NOREALLOC,