Add note about timer behavior (#279)

Description
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This PR adds a note about the user provided timer behavior when it
overflows.

Checklist:
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- [NA] I have tested my changes. No regression in existing tests.
- [NA] I have modified and/or added unit-tests to cover the code changes
in this Pull Request.

Related Issue
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#277

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source/include/core_mqtt.h

@@ -69,6 +69,14 @@ struct MQTTDeserializedInfo;
  * @note The timer should be a monotonic timer. It just needs to provide an
  * incrementing count of milliseconds elapsed since a given epoch.
  *
+ * @note As the timer is supposed to be a millisecond timer returning a 32-bit
+ * value, it will overflow in just under 50 days. But it will not cause any issues
+ * in the library as the time function is only used for calculating durations for
+ * timeouts and keep alive periods. The difference in unsigned numbers is
+ * used where unsigned wrap around is defined. Unless the timeout is bigger than
+ * 100 days (50*2) where the numbers can wrap around more than once the code
+ * should work properly.
+ *
  * @return The time elapsed in milliseconds.
  */
 typedef uint32_t (* MQTTGetCurrentTimeFunc_t )( void );