Here's a detailed explanation of -Weverything from a clang developer. He says -Weverything is intended for clang developers' own testing. He recommends using -Wall -Wextra plus any individual warnings you may want.

http://programmers.stackexchange.com/a/124574

clang does not document its warning flags like gcc does, but a long (but now incomplete) list is available here:

http://fuckingclangwarnings.com/