suzerain: #3021 improve delta3 documentation

Comment on our appreciable difference from the classic Blasius result.
Arises because we want to permit non-negligible outflow.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@43127 0bd85f0b-c151-43b9-a1e9-27231104551f