Document mlt_color_trc consumer property

Author: ddennedy

src/framework/mlt_consumer.h

@@ -88,6 +88,7 @@
  * \properties \em drop_count the number of video frames not rendered since starting consumer
  * \properties \em color_range the color range as tv/mpeg (limited) or pc/jpeg (full); default is unset, which implies tv/mpeg
  * \properties \em color_trc the color transfer characteristic (gamma), default is unset, use mlt_color_trc string values
+ * \properties \em mlt_color_trc the color transfer to use for internal processing, default is unset; use mlt_color_trc string values but only "linear" is implemented
  * \properties \em deinterlacer the deinterlace algorithm to pass to deinterlace filters, defaults to "yadif"
  */