[SERVER] Adding Equality-Sort-Range info to index documentation

The optimal index for a given query can usually be found with the equality-sort-range rule, which 

indicates that in an index, the equality fields should be first, then the sort fields, and finally the range

fields.

So, for example, let's consider this query: db.test.find({a:'x', b: {$gt: 10}}).sort(}}{{{c:1}}}

Here, `a` is an equality based match, `b` is a range based match, and `c` is used for sorting. Following 

the equality-sort-range rule, the most optimal index for the above query will be: `{a:1, c:1, b:1}`