[skip changelog] Completely document resources not inherited from referenced platform (#940)

The inherited resources resulting from a core reference are clearly documented. However, the variant reference mentioned the non-inheritance of platform.txt properties alone. For me, this left it unclear whether the lack of mention of the other resources implied that variant and tool references resulted in their inheritance, and that it was only considered necessary to document it once in the platform reference documentation.

One option for resolving this would be to remove the statement that caused the confusion. However, since this is a complex subject, I thought it better to just explicitly document the non-inheritance of the resources in both the variant and tools reference sections of the documentation to make it completely clear.

Author: per1234

docs/platform-specification.md

@@ -927,8 +927,8 @@ In the same way we can use a variant defined on another platform using the synta
     myboard.build.variant=arduino:standard
     [....]
 
-Note that referencing a variant in another platform does _not_ inherit any properties from that platform's platform.txt
-(like referencing a core does).
+Note that, unlike core references, other resources (platform.txt, bundled libraries, programmers) are _not_ inherited
+from the referenced platform.
 
 ### Tool references
 
@@ -940,6 +940,9 @@ Tool recipes defined in the platform.txt of other platforms can also be referenc
     myboard.bootloader.tool=arduino:avrdude
     [....]
 
+Note that, unlike core references, referencing a tool recipe does _not_ result in any other resources being inherited
+from the referenced platform.
+
 ### Platform Terminology
 
 Because boards can reference cores, variants and tools in different platforms, this means that a single build or upload