[Core] ROOT-9696 adjust size of TThreadedObjects automatically to IMT pool size
[Core] ROOT-9696 adjust size of TThreadedObjects automatically to IMT pool size

if the size is bigger than the default value. Document better the behaviour, too.

root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,karies/root,olifre/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root