FTS: Add examples of must, must_not and should

Add an example for each, must, must_not and should in our FTS documentation and a centralized explanation of them

similar to: https://stackoverflow.com/questions/28768277/elasticsearch-difference-between-must-and-should-bool-query

For example:
{
"must": { "conjuncts":[

{"field":"reviews.content", "match": "location"}

]},
"must_not": { "disjuncts": [

{"field":"free_breakfast", "bool": false}

]},
"should": { "disjuncts": [

{"field":"free_breakfast", "bool": true}

]}
}