MAINT: signal: Some clean up in _peak_finding.py:

* Fixed the description of the return value of argrelmin, argrelmax,
  argrelextrema and _boolrelextream.
* Fixed the example in the docstring of _boolrelextream.
* Added examples to the docstrings of argrelmin, argrelmax and
  argrelextrema.
* Added more tests for argrelmin and argrelmax.
* Removed some commented-out code and unused imports from test_peak_finding.py
  along with an unused variable (num_peaks) in _gen_gaussians().
* Removed an argument ('order=2') from the signature of
  TestArgrel.test_higher_order() in test_peak_finding.py.
* Removed unnecessary special check in argrelextrema that incorrectly
  returned a length 2 tuple when there were no relative extrema,
  regardless of the dimension of the input.  Fixes gh-2832.
* Several more minor tweaks.