Document `createIndexes` command

[rumyantseva]

What should be done?

Things to metnion:

• createIndexes called for a non-existent collection will create the collection, and the given indexes;
• createIndexes called for a non-existent field creates index for this field;
• attempt to call createIndexes for an index that already exists (when the name and the key are equal) will return index name but will not create a duplicate index.
• attempt to call createIndexes for same name and different keys or same keys but different names will return an error.

[AlekSi]

Why we want that to be documented and where?