[skip changelog] Remove inaccurate "3rd party" descriptions from documentation (#818)

The library and platform specifications apply to all libraries and platforms, including official ones.

docs/library-specification.md

-This is the specification for the 3rd party library format to be used with Arduino IDE 1.5.x onwards.
+This is the specification for the Arduino library format, to be used with Arduino IDE 1.5.x onwards.
 
 * rev.1 has been implemented starting with Arduino IDE version 1.5.3 (now superseded by rev.2)
 * rev.2 will be implemented starting from version Arduino IDE 1.5.6

docs/platform-specification.md

-This specification is a 3rd party hardware format to be used in Arduino development software starting from the Arduino IDE 1.5.x series.<br>
+This is the Arduino platform specification, for use with Arduino development software starting from the Arduino IDE 1.5.x series.<br>
 Platforms add support for new boards to the Arduino development software. They are installable either via [Boards Manager](package_index_json-specification.md) or manual installation to the *hardware* folder of Arduino's sketchbook folder.<br>
 A platform may consist of as little as a single configuration file.