Online documentation for Web Dispatcher is found on help.sap.com.
The current path to the documentation (version 7.4x) is
help.sap.com =>
SAP NetWeaver =>
SAP NetWeaver 7.4 =>
Application Help =>
Function-Oriented View <Langauge> =>
Application Server =>
Application Server Infrastructure =>
Components of SAP NetWeaver Application Server ABAP =>
SAP Web Dispatcher.
Direct link: Web Dispatcher Documentation English and Web Dispatcher Documentation German
Documentation for version 7.2x:
help.sap.com =>
SAP NetWeaver =>
SAP NetWeaver 7.3 =>
Application Help => SAP Library =>
Administration Information =>
Technical Operations for SAP Netweaver =>
Administration of Standalone Engines =>
Technical Operations of SAP Web Dispatcher.
The direct link is here