gsignal: Mention handler ID type in signal connection macro docs
gtk-doc doesn’t make the return type clear, because these are macros rather than inline functions, so people often have to guess at the return type (or look it up from g_signal_connect_closure(), but that’s hard work). Make it clear that the return type for handler IDs is gulong. While there, fix the capitalisation of ‘id’ to ‘ID’ in a few places.
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