Tidy and document the `inline`/`__inline`/`__inline__` situation.

We can (and should) use `inline` in C++ but not in C headers that
need to support ANSI/C89/C90. `__inline__` works everywhere, but
traditionally we've used the BSD `__inline` in public headers.

Change-Id: I920e0a1f5fcd01cf8ddd89240ef66b25c50378d7
diff --git a/libc/include/bits/threads_inlines.h b/libc/include/bits/threads_inlines.h
index 5878e0a..459866e 100644
--- a/libc/include/bits/threads_inlines.h
+++ b/libc/include/bits/threads_inlines.h
@@ -124,7 +124,7 @@
 };
 #pragma clang diagnostic pop
 
-static inline void* _Nonnull __bionic_thrd_trampoline(void* _Nonnull __arg) {
+static __inline void* _Nonnull __bionic_thrd_trampoline(void* _Nonnull __arg) {
   struct __bionic_thrd_data __data =
       *__BIONIC_CAST(static_cast, struct __bionic_thrd_data*, __arg);
   free(__arg);