Revise docstring of convert case styles methods
Revise docstring of convert case styles methods

Revise docstring of convert_camelcase_to_snakecase() method and
convert_snakecase_to_camelcase() method to make it clear about the
behavior.

Change-Id: I3ea50426dc3c873d7b73553dab4b10cef52535c8
Closes-Bug: #1890078

stackforge/tacker,stackforge/tacker,openstack/tacker,openstack/tacker,openstack/tacker