Fix aws_thread and aws_condition_variable header doc

[nchong-at-aws]

1. In thread.h the doc for aws_thread_clean_up currently reads:

/**
 * Cleans up the thread handle. Either detach or join must be called
 * before calling this function.
 */

But the function itself detaches the thread (if it is currently running). There is no function to detach a thread and calling aws_thread_join before this function makes it a no-op.

In condition_variable.h:

2. The docs for aws_condition_variable_wait, aws_condition_variable_wait_pred, aws_condition_variable_wait_for, and aws_condition_variable_wait_for_pred should note that the function must be called with the mutex locked by the calling thread otherwise the behavior is undefined.

3. Additionally, the docs for aws_condition_variable_wait and aws_condition_variable_wait_for should note that spurious wakeups can occur: i.e., the return of the function does not apply anything about the wakeup condition associated with the variable. To avoid this problem the user can use the _pred versions of these functions or re-test the condition explicitly.

[jmklix]

1. Fixed with this PR: Part 1 of aws_thread_clean_up fix and contract clarification #814
   2 & 3 will be fixed with this PR: improve condition_variable documentation