Clarify index miss % in mongostat manual

Author: dcrosta

source/reference/mongostat.txt

@@ -217,7 +217,8 @@ average operations per second.
 
 .. describe:: idx miss
 
-   The percent of index (btree page) misses. This is a sampled value.
+   The percent of index access attempts that required a page fault
+   to load a btree node. This is a sampled value.
 
 .. describe:: qr