MAINT: Changed class constructor __init__ GL08 reporting

[mattgebert]

Changed validate function in validate.py for empty docstring error GL08 ("The object does not have a docstring"), specifically for __init__ constructors. If an empty docstring for __init__, will first check if class docstring is satisfied for parameter mismatches, before deciding to report GL08 (if mismatches exist, i.e. incomplete documentation).

Implemented new docstring test classes in test_validate.py to verify correct functionality.
Also updated readthedocs format.rst and validation.rst to reflect new changes to constructor checking behavior.

Solves #591.

Note, I think a modification to the readthedocs https://numpydoc.readthedocs.io/en/latest/validation.html#ignoring-validation-checks-with-inline-comments ought to be made

P.s. First time pull requesting to a public repo, let me know if anything is insufficient.

[larsoner]

Had a quick look at the code and this looks reasonable to me!

Note, I think a modification to the readthedocs https://numpydoc.readthedocs.io/en/latest/validation.html#ignoring-validation-checks-with-inline-comments ought to be made

Sounds good, do you want to add it here?