Add SDK docs on native getters as its not self-explanatory

[petreze]

Problem

Currently in the JS SDK we are providing native getters instead of getX methods:

/**
 * @returns {?Key}
 */
get submitKey() {
    return this._submitKey;
}

Users of the JS SDK does not expect this approach and they assume that they should use getSubmitKey() instead of typing only submitKey for example

Solution

Add explanation/example in the docs in order to be more convenient for the users

Alternatives

No response