Improve the definition of FQBN and explicitly state which characters are allowed

Author: MatteoPologruto

Diff for: docs/FAQ.md

@@ -9,7 +9,11 @@ When you run [`arduino-cli board list`][arduino cli board list], your board does
 
 ## What's the FQBN string?
 
-For a deeper understanding of how FQBN works, you should understand the [Arduino platform specification][0].
+FQBN stands for Fully Qualified Board Name. It has the following format:
+`VENDOR:ARCHITECTURE:BOARD_ID[:MENU_ID=OPTION_ID[,MENU2_ID=OPTION_ID ...]]`, with each `MENU_ID=OPTION_ID` being an
+optional key-value pair configuration. Each field accepts letters (`A-Z` or `a-z`), numbers (`0-9`), underscores (`_`),
+dashes(`-`) and dots(`.`). The special character `=` is accepted in the configuration value. For a deeper understanding
+of how FQBN works, you should understand the [Arduino platform specification][0].
 
 ## How to set multiple board options?