man page: Clearer documentation for config file search order
man page: Clearer documentation for config file search order

This documents the config files in the same order that stbt will search
them. It's clearer especially because $STBT_CONFIG_FILE is also searched
first to last.

LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester