[doc] Document the format tag without a \param

[karpovantonme]

Both read_image_info overloads document a tag parameter, and the argument it refers to has no name:

/// \param file It's a device. Must satisfy is_adaptable_input_device metafunction.
/// \param tag  Defines the image format. Must satisfy is_format_tag metafunction.
template <typename Device, typename FormatTag>
auto read_image_info(Device& file, FormatTag const&, ...)

Doxygen cannot attach a \param to an unnamed argument, so it reports tag as a parameter that does not exist and the format tag ends up undocumented either way.

Two ways to fix it, and I picked the second:

• name the argument tag. It is never used in the body, so this would produce an unused-parameter warning, and every other FormatTag argument across io/ is unnamed as well, so it would also break with the surrounding style;
• keep the declaration and move the sentence into the description. That is what this does.

The wording is the same as before, only the position changed.

[mloskot]

LGTM, thanks!