*Custom*, not *customer*

Argh.
Start documenting Account Summary report

Help: add documentation for Account Summary report in a structured
format. In particular, document the `Accounts' tab of the report's
options dialog box.

The documentation for each report will take up a hefty amount of
vertical space. I'm therefore planning to split out coverage of each
report into a separate DocBook XML file and include it in the reports
chapter using a DocBook entity, just as the chapters themselves are
included.