c/fields.desc: Clarify some vcs-* related tags

Clarify that vcs-field-not-canonical is based on Alioth giving 301
HTTP status codes on "non-canonical" URIs.  Also mention that the tag
(and vcs-field-bitrotted) are based on "data lists" rather than
actually sending HTTP requests.
  In theory, the latter should be redundant given Lintian's design
criteria, but it is probably best to assume that users will not read
the (full) documentation.

Signed-off-by: Niels Thykier <niels@thykier.net>