[clang-tidy][NFC] Clarify switch-missing-default-case doc (#164699) (#164709)

Falling through a defaultless switch statement has well defined
behavior. Fixes https://github.com/llvm/llvm-project/issues/164699.

Credit for noticing this problem goes to user "pozz" on comp.lang.c,
Message-ID: <10da67g$3q59f$1@dont-email.me>
GitOrigin-RevId: b474be668091301d4a214da922f0cb98d416dc6b
1 file changed