Fix comments and enforce expected return values

Improve calibrate and add_context usage() printout. Document and enforce
the expected return values for the cmd_calibrate/cmd_addcontext chain of
functions.

Fix add_context usage()

Add documentation in liblttng-ctl.

Signed-off-by: Daniel Thibault <daniel.thibault@drdc-rddc.gc.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>