kernel: Fix typos in documentation.

Spotted by Clang (yes, there is a -Wdocumentation that understands
how to parse Doxygen comments!)

src/system/kernel/debug/user_debugger.cpp

@@ -894,7 +894,7 @@ user_debug_post_syscall(uint32 syscall, void *args, uint64 returnValue,
 /**	\brief To be called when an unhandled processor exception (error/fault)
  *		   occurred.
  *	\param exception The debug_why_stopped value identifying the kind of fault.
- *	\param singal The signal corresponding to the exception.
+ *	\param signal The signal corresponding to the exception.
  *	\return \c true, if the caller shall continue normally, i.e. usually send
  *			a deadly signal. \c false, if the debugger insists to continue the
  *			program (e.g. because it has solved the removed the cause of the

src/system/kernel/thread.cpp

@@ -2865,15 +2865,14 @@ thread_block()
 
 /*!	Blocks the current thread with a timeout.
 
-	The thread is blocked until someone else unblock it or the specified timeout
-	occurs. Must be called after a call to thread_prepare_to_block(). If the
-	thread has already been unblocked after the previous call to
+	The current thread is blocked until someone else unblock it or the specified
+	timeout occurs. Must be called after a call to thread_prepare_to_block(). If
+	the thread has already been unblocked after the previous call to
 	thread_prepare_to_block(), this function will return immediately. See
 	thread_prepare_to_block() for more details.
 
 	The caller must not hold the scheduler lock.
 
-	\param thread The current thread.
 	\param timeoutFlags The standard timeout flags:
 		- \c B_RELATIVE_TIMEOUT: \a timeout specifies the time to wait.
 		- \c B_ABSOLUTE_TIMEOUT: \a timeout specifies the absolute end time when