No html output from config doc build

I made a simple change to libs/config/doc/macro_reference.qbk, then ran b2 from
the command line with no arguments. Although the quickbook.exe build seemed to
go OK, no html output was produced.

There were several warnings:

warn: Unable to construct ./standalone
...
config.qbk: warning: Invalid attributes for 'article document info': purpose,
category, dirname
guidelines.qbk:131: warning: line breaks generate invalid boostbook (will only
note first occurrence).
...updated 70 targets...

Any ideas?

--Beman