[instance-locator] use instance reference instead of pointer (#2007)

This commit changes the `InstanceLocator` class to keep track of a
reference to `otInstance` (instead of a pointer) to make it behave
similar to other `ObjectLocator` classes. The method `GetInstance()`
in all locator objects is updated to provide a reference (instead
of a pointer) to `otInstance`.

The logging macros are updated such that a reference to `otInstance`
is passed as the first argument (with the exception of
`otLog<Level>Plat()` macros which are used by platform code in C
domain). The documentation for log macros are also updated.

tests/unit/test_platform.cpp

@@ -82,19 +82,19 @@ otInstance *testInitInstance(void)
     otInstance *instance = NULL;
 
 #if OPENTHREAD_ENABLE_MULTIPLE_INSTANCES
-    size_t instaneBufferLength = 0;
+    size_t instanceBufferLength = 0;
     uint8_t *instanceBuffer = NULL;
 
     // Call to query the buffer size
-    (void)otInstanceInit(NULL, &instaneBufferLength);
+    (void)otInstanceInit(NULL, &instanceBufferLength);
 
     // Call to allocate the buffer
-    instanceBuffer = (uint8_t *)malloc(instaneBufferLength);
+    instanceBuffer = (uint8_t *)malloc(instanceBufferLength);
     VerifyOrQuit(instanceBuffer != NULL, "Failed to allocate otInstance");
-    memset(instanceBuffer, 0, instaneBufferLength);
+    memset(instanceBuffer, 0, instanceBufferLength);
 
     // Initialize OpenThread with the buffer
-    instance = otInstanceInit(instanceBuffer, &instaneBufferLength);
+    instance = otInstanceInit(instanceBuffer, &instanceBufferLength);
 #else
     instance = otInstanceInitSingle();
 #endif