Extent TRefArray doc to point to the size limitation.
Extent TRefArray doc to point to the size limitation.

This addresses ROOT-6130

(cherry picked from commit 70cfcbc3c07d8b02c2a737ca4509e216f040e7e0)

tc3t/qoot,Dr15Jones/root,tc3t/qoot,Dr15Jones/root,tc3t/qoot,tc3t/qoot,kirbyherm/root-r-tools,kirbyherm/root-r-tools,kirbyherm/root-r-tools,tc3t/qoot,Dr15Jones/root,Dr15Jones/root,kirbyherm/root-r-tools,Dr15Jones/root,tc3t/qoot,tc3t/qoot,tc3t/qoot,kirbyherm/root-r-tools,kirbyherm/root-r-tools,tc3t/qoot,Dr15Jones/root,tc3t/qoot,kirbyherm/root-r-tools,Dr15Jones/root