Repeat `header_loader` deprecation in API doc for clarity
Repeat `header_loader` deprecation in API doc for clarity

nfvs/flask-login,nfvs/flask-login,neilsh/flask-login,maxcountryman/flask-login,neilsh/flask-login,HoustonFortney/flask-login,HoustonFortney/flask-login,alanhamlett/flask-login