docs: kernel-doc: properly recognize parameter lines with colons

The previous attempt to properly handle literal blocks broke parsing of
parameter lines containing colons; fix it by tweaking the regex to
specifically exclude the "::" pattern while accepting lines containing
colons in general.  Add a little documentation to the regex while in the
neighborhood.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 8d295fba ("kernel-doc: better handle '::' sequences")
Signed-off-by: Jonathan Corbet <corbet@lwn.net>