Update description.md

Moving generally applicable information from the Python specific addendum as per https://forum.exercism.org/t/moving-generic-instructions-from-python-addendum-to-overall-problem-specification/13678/1 and https://forum.exercism.org/t/square-root-exercise-instructions-could-be-clearer/13664/11

See also https://github.com/exercism/python/pull/3815/files

exercises/square-root/description.md

@@ -9,5 +9,7 @@ Check out the Wikipedia pages on [square root][square-root] and [methods of comp
 
 Recall also that natural numbers are positive real whole numbers (i.e. 1, 2, 3 and up).
 
+It is possible to compute the square root of any natural number using only natural numbers in the computation.
+
 [square-root]: https://en.wikipedia.org/wiki/Square_root
 [computing-square-roots]: https://en.wikipedia.org/wiki/Methods_of_computing_square_roots