kernel: spinlock: update k_spin_lock() documentation wrt context switch

Threads must not attempt to context switch if they are holding a spinlock.

Add this information to the documentation for k_spin_lock().

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>

Author: mathieuchopstm

include/zephyr/spinlock.h

@@ -175,6 +175,9 @@ static ALWAYS_INLINE void z_spinlock_validate_post(struct k_spinlock *l)
  * in uniprocessor contexts such that the locking reduces to an
  * interrupt mask operation.
  *
+ * @warning
+ * Holding a spinlock when a context switch occurs is illegal.
+ *
  * @param l A pointer to the spinlock to lock
  * @return A key value that must be passed to k_spin_unlock() when the
  *         lock is released.