[doc] Drop the dangling \param from empty_formatter

[karpovantonme]

empty_formatter takes its argument only to deduce a type, so the argument has no name:

    \param - container used to select a correct value_type for the
                 resulting empty_container<>.
    ...
    empty_formatter(const RangeT&)

Doxygen has nothing to bind that \param to, so the description does not reach the generated page. The dash standing in for the name is not a name either. It is also the only one of its kind: 1 of 725 \param tags in this library, and I did not find another in the 44 other Boost libraries I have checked out

So the explanation moves into the description text, where it is not tied to a name, and the argument gets a commented name the way string/detail/replace_storage.hpp:65 already writes it:

    empty_formatter(const RangeT& /*Input*/)

And no behaviour change, and no unused-parameter warning added for anyone building with -Wextra

So two siblings in the same header, const_formatter(const RangeT& Format) and dissect_formatter(const FinderT& Finder), both name their argument and both document it correctly, so this was the odd one out

Found with a checker I wrote with help from Claude Opus 5, and read by hand before proposing

[nigels-com]

Looks good.