Mention in docs that registry files must use POSIX compatible file names

[leouieda]

Page that contains the error or needs to be improved (paste link): https://www.fatiando.org/pooch/latest/registry-files.html and https://www.fatiando.org/pooch/latest/api/generated/pooch.Pooch.html#pooch.Pooch.load_registry (docstring of Pooch.load_registry)

Description of the problem/suggestion:

As pointed out in #357, the files names in registry files must now escape any special characters or spaces. This was needed so that spaces in file names could be supported in #315. This behavior will break any existing registry files that didn't have escaped quotations (which was a bug since we should have been requiring escaped characters from the start).

We need to add a note to the pages above explaining this requirement and that it was introduced in Pooch 1.7.