cleanup the bits headers

src/header/bits_key-t/cbindgen.toml

@@ -1,5 +1,7 @@
 # POSIX header spec: https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/sys_types.h.html
 #
+# This type is split out to prevent importing all of sys/types.h into other headers.
+#
 # POSIX headers that require key_t:
 # - sys/ipc.h
 # - sys/types (where it should be defined)

src/header/bits_key-t/mod.rs

@@ -1 +1,2 @@
+/// Used for XSI interprocess communication.
 pub type key_t = core::ffi::c_int;