context: add a comment explaining why we are not adding bees_unreadahead

At the end of scanning one extent, in theory we do not need that extent
any more.  In practice, it hurts benchmark scores if we drop the extents
after reading them.

Add a comment to note this where we put the bees_unreadhead call.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-context.cc

@@ -717,6 +717,8 @@ BeesContext::scan_one_extent(const BeesFileRange &bfr, const Extent &e)
 		BEESLOGINFO("scan: " << pretty(e.size()) << " " << to_hex(e.begin()) << " [" << bar << "] " << to_hex(e.end()) << ' ' << name_fd(bfr.fd()));
 	}
 
+	// Costs 10% on benchmarks
+	// bees_unreadahead(bfr.fd(), bfr.begin(), bfr.size());
 	return bfr;
 }