Improvements like...
- fixing typos,
- removing trailing and double whitespaces,
- using `jldoctest` blocks where applicable,
- fixing, updating or correctly setting up existing doctests,
- improving consistency (for example, always use "# Examples" instead
of other variants),
- removing empty lines between docstrings and functions,
- instead of mentioning keywords, put them into the docstring,
- adding some missing but useful keywords,
- adding references (`@ref`),
- using LaTeX math where applicable, and
- linking papers.
Debatable stuff that is untouched:
- BE/AE s/z irregularities ("normalise" versus "normalize") since
most papers use the AE version while the Flux source code was
written with BE spelling.
- Names of normalization functions are capitalized
("Batch Normalization" instead of "batch normalization").
It's annoying that when third party servers go down our tests break.
Let's at least make sure that if our tests break due to server outages
it's our fault.
