I'm not sure why the ui5 team is not adding the properties with the methods.But if you go through any one of the set or get method you can get that property like,
setInitialPage(oInitialPage): sap.m.NavContainer
Setter for the associated initialPage.
And also if you think why that properties also required
if you know property or variable is declared by small letters. and if you do setter getter for that variable getInitialPage(). 1st character of method is small and next letter is big. then easily you can identify all the properties with those methods names.
Also the API is provided methods is enough to identify the property. Again extra properties with other section is not make sense.
what do you say??