from the hg man page..

> x::y
>    A DAG range, meaning all changesets that are descendants of x and
>    ancestors of y, including x and y themselves. If the first endpoint
>    is left out, this is equivalent to ancestors(y), if the second is left
>    out it is equivalent to descendants(x).
>
> descendants(set)
>    Changesets which are descendants of changesets in set.

so "x::" is the same as "descendants(x)", which is "descendants of x including x itself".

what's a descendant exactly according to the glossary?

> Descendant
>    Any changeset that can be reached by a chain of child changesets from
>    a given changeset.  More precisely, the descendants of a changeset can
>    be defined by two properties: the child of a changeset is a descendant,
>    and the child of a descendant is a descendant.

this definition indicates that a descendant does not include the changeset itself, which is in conflict with what descendants() does.