docs: Update description for dwaCompressionLevel (#5074)

Clarify in the documentation that `openexr:dwaCompressionLevel` is a
read-only attribute and cannot be used to set the compression level on
write.

Not sure if this is an obvious change or not, but it wasn't immediately
clear if the attribute is meant to be writable.

Signed-off-by: Aamir Raza <aamir_raza@outlook.com>

Author: Aamir-Raza

src/doc/builtinplugins.rst

@@ -1608,7 +1608,9 @@ The official OpenEXR site is http://www.openexr.com/.
      - the MIPmap rounding mode of the file.
    * - ``openexr:dwaCompressionLevel``
      - float
-     - compression level for dwaa or dwab compression (default: 45.0).
+     - compression level for dwaa or dwab compression (default: 45.0). 
+       Reflects the level used when reading an existing file. To set 
+       the compression level, use the ``compression`` attribute.
    * - ``openexr::luminancechroma``
      - int
      - If nonzero, indicates whether the image is a luminance-chroma image.