Currently url-hexify-string has an optional argument allowed-chars,
which is a long vector. However, it is more natural to specify a list of
characters. Internally, the default value of allowed-chars is specified
by converting a list of chars to such a vector by using an internal
function url--allow-chars. I would like to suggest to document
url--allow-chars for external use, thus we can improve the interface of
the url-hexify-string function.

I can draft the document change, if you agree with me.