DOC: note about query strings in HDFStore selection
closes #6901

See #6901. That was the reported error, so I wanted to make that example explicit. It will fail with a syntax error if you interpolate the string into a query
 I'll point out the single and double quote
 maybe show that u can pass a string if u surround it with quotes 
 done
 turning out to be more controversial than i would've expected
 the locals interpolation is a bit magical (but useful)

I always use direct quotes -
 speaking of that, i must've overlooked this when refactoring local vars API but shouldn't pytables follow that convention?
 it should follow the same convention
problem?
 i meant with the `@` syntax. ... that doesn't seem to work
 in the example from #6901 
 hmm
ok so is this more like eval or query?
it doesn't have context so maybe should be more like eval in that locals need @ ?
 i'll make a new issue, merge this doc PR?
 yep
