Log Message:
Updated gnucash-docs to 2.6.13.

Concurrent with the release of Gnucash 2.6.13 we're pleased to also release a \ 
new version 2.6.13 of the companion Help and Tutorial and Concepts Guide.

    Fix the very stale Release Schedule link in the Guide.
    Bug 637004 - Document Payable/Receivable Aging Rpts and new options for \ 
allowing customer and vendor address info to display.
    Fix unclear statement re docbook stylesheets in README.
    Bug 712764 - Clarify Account type designations in the Tutorial.
    Bug 751686 - Explain how to use Find Transaction to report per-employee.
    Bug 754239 - Fix order of Report menu items.
    Bug 754239 - Update Report menu item names to Save Report Configuration.
    Bug 765131 - Document Investment Return of Capital in the Guide.