stbt.*_motion: default parameters for motion testing are set in stbt.conf

Much like the templatematching functions, the parameters for `detect_motion`
and `wait_for_motion` can now be set in the users STBT_CONFIG_FILE (or else
are read from the system installed config file).

This gives users flexibility, particularly for the `noise_threshold`
parameter whose value should depend on the quality of the video source.
For example, an analogue source will contain more incidental noise than
HD video, and therefore a lower `noise_threshold` value should be used.
Being able to set a global parameter which is then applied to every call
to the motion detection algorithm (unless overridden) means that a custom
value does not need to be applied to every in-script call explicitly.

[Committer's note: The motivation is that we have seen several
`wait_for_motion` calls raise `MotionTimeout` when there was in fact
motion. We want to be able to change the default `noise_threshold` and
`consecutive_frames` without breaking backwards compatibility with other
users of stb-tester.]
detect_motion/wait_for_motion: reformat docstrings to use definition list

The parameter names are now subsection headings, along with their default
values. Some line wrapping was needed because of the extra indentation.

Note that the docs for `mask` were altered to match the format of
`consecutive_frames` and `noise_threshold`, although its default value is
not placed into the README dynamically because it does not come from
stbt.conf.
wait_for_motion, detect_motion: Document section of config file

...from where we read the default values for the `noise_threshold` and
`consecutive_frames` parameters. It wasn't documented that these values
belong in the `[motion]` section of the config file.
api-doc.sh: Cleanup: Consistent, unabbreviated function name

"sub" has a meaning of its own; "substitute" is consistent with the
function "substitute_python_docstring" in the same script.
stbt.wait_for_motion: Cleanup: Remove duplicate default value lookup

`wait_for_motion` only uses `noise_threshold` to pass it to
`detect_motion`; `detect_motion` already looks up `noise_threshold` in
the configuration file if the passed-in argument is `None`.
Merge branch 'motion-parameters-from-config'

* motion-parameters-from-config:
  stbt.wait_for_motion: Cleanup: Remove duplicate default value lookup
  api-doc.sh: Cleanup: Consistent, unabbreviated function name
  wait_for_motion, detect_motion: Document section of config file
  detect_motion/wait_for_motion: reformat docstrings to use definition list
  stbt.*_motion: default parameters for motion testing are set in stbt.conf