The detailed information can be found under Offline Lattice Diamond Help.
To access the Offline Lattice Diamond Help page, open your Diamond GUI interface then choose "Lattice Diamond Help" from the Help menu.
After that, go to the path described below to locate the information for prj_src:
Tcl Command Reference Guide > Diamond Tool Tcl Command Syntax > Diamond Project Tcl Commands
Other commands are also covered on this help page.