bug#20426: browse-url-XYZ should mention the XYZ browser in the prompt

[積丹尼 Dan Jacobson]

X-Debbugs-Cc: Katsumi Yamaoka <yamaoka@jpl.org>

Functions like

   (defun browse-url-chromium (url &optional _new-window)
     "Ask the Chromium WWW browser to load URL.
   Default to the URL around or before point.  The strings in
   variable `browse-url-chromium-arguments' are also passed to
   Chromium."
     (interactive (browse-url-interactive-arg "URL: "))

should each prompt differently, instead of just the hardwired "URL: ",
else the user can't be sure he really called the right function.