Nothing is using StrategyNotifyBgWriter() anymore

While browsing through some of the clock-sweep code I noticed that the 
only place StrategyNotifyBgWriter() is called now is in 
BackgroundWriterMain()[1]. Presumably this isn't what's desired. If 
nothing else, it means the function's description comment is wrong, as 
are comments in BackgroundWriterMain(). This isn't new; 9.2 shows the 
same thing and that's when the function was added. I'm not sure what the 
right fix here is, since ISTM joggling bgwriter for every call to 
BufferAlloc() would be overkill.


1: 
https://doxygen.postgresql.org/freelist_8c.html#aabbd7d3891afc1d8531c3871d08d4b28