[boost] [proto v4 docs]Suggest warning about transform composition failure

The attached fails to compile because when the output of one when transform is immediately fed to another transform, the compiler complains because it expects Expr& instead of Expr as 1st arg to the composition. Although the docs on: libs/proto/doc/html/boost_proto/appendices/rationale/result_of.html *hint* that this could happen, it's not obvious. It would have helped me to see this on that page. IOW, that page could specifically say that transform compositions are not possible because a transform expects Expr& and when transforms are composed without storing intermediate values in a variable, then the 1st transform produces an Expr and not an Expr&; so, the second transform rejects it.