Refine comments on closed caption test methods
Refine comments on closed caption test methods

This change refines and emphasizes the message of the test only
methods for closed captions in text engine.

Change-Id: I0f13f0fa48f3b464c8dc4166ddbd12f09f53874c

shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player