Clarify test comments.

Author: Vytautas Astrauskas

tests/compile-fail/concurrency/libc_pthread_join_multiple.rs

@@ -1,6 +1,6 @@
 // ignore-windows: Concurrency on Windows is not supported yet.
 
-// Joining the same thread multiple times is undefined behavior.
+// Joining the same thread from multiple threads is undefined behavior.
 
 #![feature(rustc_private)]
 

tests/run-pass/concurrency/tls_lib_drop.rs

@@ -2,6 +2,9 @@
 
 //! Check that destructors of the library thread locals are executed immediately
 //! after a thread terminates.
+//!
+//! FIXME: We should have a similar test for thread-local statics (statics
+//! annotated with `#[thread_local]`) once they support destructors.
 
 #![feature(thread_local_internals)]