[docs] specify seconds as the unit for child timeout value (#3145)

include/openthread/thread.h

@@ -281,7 +281,7 @@ OTAPI bool OTCALL otThreadIsDiscoverInProgress(otInstance *aInstance);
  *
  * @param[in]  aInstance A pointer to an OpenThread instance.
  *
- * @returns The Thread Child Timeout value.
+ * @returns The Thread Child Timeout value in seconds.
  *
  * @sa otThreadSetChildTimeout
  */
@@ -291,7 +291,7 @@ OTAPI uint32_t OTCALL otThreadGetChildTimeout(otInstance *aInstance);
  * Set the Thread Child Timeout used when operating in the Child role.
  *
  * @param[in]  aInstance A pointer to an OpenThread instance.
- * @param[in]  aTimeout  The timeout value.
+ * @param[in]  aTimeout  The timeout value in seconds.
  *
  * @sa otThreadSetChildTimeout
  *

src/core/thread/mle.hpp

@@ -798,7 +798,7 @@ public:
     /**
      * This method returns the MLE Timeout value.
      *
-     * @returns The MLE Timeout value.
+     * @returns The MLE Timeout value in seconds.
      *
      */
     uint32_t GetTimeout(void) const { return mTimeout; }
@@ -806,6 +806,8 @@ public:
     /**
      * This method sets the MLE Timeout value.
      *
+     * @param[in]  aTimeout  The Timeout value in seconds.
+     *
      */
     otError SetTimeout(uint32_t aTimeout);
 

src/ncp/spinel.h

@@ -1088,6 +1088,7 @@ typedef enum {
 
     /// Thread Child Timeout
     /** Format: `L`
+     *  Unit: Seconds
      *
      *  Used when operating in the Child role.
      */