added documentation for featuretypecachesize

I added some more information regarding the featuretypecachesize parameter. I think its helpful to give the user advice what this parameter may cause when it is set to a too low value (lower than the number of all featuretypes). 
I also added information about the default value, but i am not absolutely sure with this one.
Hints i have found:
https://github.com/geoserver/geoserver/blob/master/src/main/src/main/java/org/geoserver/catalog/ResourcePool.java#L152
It would also be useful to know how a value of 0 will behave here.