[skip changelog] Document sketch filename requirements (#1287)

Previously, the Arduino Sketch Specification only documented the requirements for sketch folder names. Since the primary
sketch filename must match the folder name, this appeared at a glance to be self-evident. However, sketches may consist
of multiple files, and so the specification was ambiguous regarding the naming requirements for additional files.

Author: per1234

docs/sketch-specification.md

@@ -5,17 +5,17 @@ The programs that run on Arduino boards are called "sketches". This term was inh
 
 ## Sketch folders and files
 
+The sketch root folder name and code file names must start with a basic letter (`A`-`Z` or `a`-`z`) or number (`0`-`9`),
+followed by basic letters, numbers, underscores (`_`), dots (`.`) and dashes (`-`). The maximum length is 63 characters.
+
+Support for names starting with a number was added in Arduino IDE 1.8.4.
+
 ### Sketch root folder
 
 Because many Arduino sketches only contain a single .ino file, it's easy to think of that file as the sketch. However,
 it is the folder that is the sketch. The reason is that sketches may consist of multiple code files and the folder is
 what groups those files into a single program.
 
-The sketch root folder name must start with a basic letter (`A`-`Z` or `a`-`z`) or number (`0`-`9`), followed by basic
-letters, numbers, underscores (`_`), dots (`.`) and dashes (`-`). The maximum length is 63 characters.
-
-Support for sketch folder names starting with a number was added in Arduino IDE 1.8.4.
-
 ### Primary sketch file
 
 Every sketch must contain a `.ino` file with a file name matching the sketch root folder name.