Bug#688481: [copyright-format] Clarify that trailing slashes in directory names are invalid.

[Nicolas Boulenguez <nicolas@debian.org>]

Package: debian-policy
Followup-For: Bug #688481

Please also make explicit that the space character cannot be escaped,
and how to select a path containing a space. Suggestion:

  This syntax does not distinguish file names from directory names; a
  trailing slash in a pattern will never match any actual path. A
  whole directory tree may be selected with a pattern like "foo/*".

  The space character, used to separate patterns, cannot be escaped
  with a backslash. A path like "foo bar" may be selected with a
  pattern like "foo?bar".