QDoc: Allow \fn commands to be split over multiple lines

QDoc allows user to document a callable element by usage of an "\fn" topic command.

Generally, an "\fn" commands accepts an argument, that spans up to the end of the line with some small exceptions, that more or less encodes the information that QDoc needs to understand which element the user is trying to document.

For C++ methods/function, the information can be quite verbose. Especially as QDoc supports more of the C++ concepts.

For example, a recent change now requires the user to provide information about template declarations for C++ callables (QTBUG-118080).

Due to the amount of information required, forcing the argument to be laid out on a single line makes an "\fn" command not only difficult to read and identify, but difficult to maintain.

Hence, QDoc should allow the user to lay out the argument for an "\fn" command over multiple lines, to simplify both writing and reading the instances of the commands that are information dense.