Fields doc: add in simple documentation for the field.Flags and field.Label classes.
Fields doc: add in simple documentation for the field.Flags and field.Label classes.

Aaron1992/wtforms,crast/wtforms,Aaron1992/wtforms,wtforms/wtforms,jmagnusson/wtforms,subyraman/wtforms,pawl/wtforms,cklein/wtforms,skytreader/wtforms,Xender/wtforms,hsum/wtforms,pawl/wtforms