[skip changelog] Always mention "user directory" along with "sketchbook" in documentation (#816)

Arduino IDE and Arduino Web Editor use the term "sketchbook", while Arduino CLI and Arduino Pro IDE use the term "user directory". In order for the documentation to be friendly to all users of Arduino's development software, both terms must be used in the documentation.

docs/library-specification.md

@@ -185,7 +185,7 @@ A hypothetical library named "Servo" that adheres to the specification follows:
 
 ## Working with multiple architectures
 
-Libraries placed in the user’s sketchbook folder (in the libraries/ subfolder) will be made available for all boards, which may include multiple different processor architectures. To provide architecture-specific code or optimizations, library authors can use the `ARDUINO_ARCH_XXX` preprocessor macro (`#define`), where XXX is the name of the architecture (as determined by the name of the folder containing it), e.g. `ARDUINO_ARCH_AVR` will be defined when compiling for AVR-based boards. For example,
+Libraries placed in the `libraries` subfolder of the sketchbook folder (AKA "user directory") will be made available for all boards, which may include multiple different processor architectures. To provide architecture-specific code or optimizations, library authors can use the `ARDUINO_ARCH_XXX` preprocessor macro (`#define`), where XXX is the name of the architecture (as determined by the name of the folder containing it), e.g. `ARDUINO_ARCH_AVR` will be defined when compiling for AVR-based boards. For example,
 
     #if defined(ARDUINO_ARCH_AVR)
       // AVR-specific code

docs/platform-specification.md

 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>
+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 (AKA "user directory").<br>
 A platform may consist of as little as a single configuration file.
 
 ## Hardware Folders structure