Spurious warning

[vinniefalco]

This produces a warning when you write [br] in a QBK file:

quickbook/src/actions.cpp

Line 265 in 5f2e8c2

<< "line breaks generate invalid boostbook "

Thing is... it works fine. There is no other "boostbook processor" on the horizon, so the warning is rather pointless. We want to make doc warnings into errors in our CI builds but this is in the way.