docs: Fix UploadVerifyUpload/UploadStashFile hook docs

docs: Fix UploadVerifyUpload/UploadStashFile hook docs

1. FSFile::getPropsFromPath() is not used by any code any more.
https://codesearch.wmflabs.org/search/?q=FSFile%3A%3AgetPropsFromPath&i=1
The only remaining usage is in one test. We might as well remove the
function.

2. The $props array is passed to the hook for convenience, in case all
the file properties are already available. Fetching them from a file on
disc can be an expensive operation, and should be avoided if the
information is already available. But the caller does not guarantee this
is set. Other callees already know this can be a falsy value, notably
LocalFile::upload().

Change-Id: I43724d18467b6fb68a963b2206332cf553c81b2c