[DOCS] nested field datatypes def in FAQ is incorrect

[jrodewig]

The ECS FAQ notes that the nested field datatype is not used. However, the definition of the nested field datatype is incorrect:

ECS does not use nested datatypes, which are arrays of objects.

An object datatype, which is used by ECS, can also contain an array of objects as a value. The primary difference between the two is that objects in a nested datatype can be queried independently.

However, I'm not sure we need to go into that level of detail here. It would likely suffice to say:

ECS does not use nested datatypes.