keyboardModifiers: add notes to ambiguous keys

In Language/Functions/USB/Keyboard/keyboardModifiers.adoc, add
explanatory notes for keys with ambiguous names:

- KEY_{LEFT,RIGHT}_GUI are the OS logo keys, "command" on Mac
- KEY_{LEFT,RIGHT}_ALT is "option" on Mac
- KEY_RIGHT_ALT can be labeled "AltGr".

Author: edgar-bonet

Language/Functions/USB/Keyboard/keyboardModifiers.adoc

@@ -36,16 +36,16 @@ The available macros are listed below:
 These are eight keys within the alphanumeric cluster of the keyboard that are meant to modify the normal action of another key when the two are pressed in combination. Whereas Shift and AltGr are used to access extra characters, the others are mostly used for keyboard shortcuts.
 
 |===
-|Key	|Hexadecimal value	|Decimal value
-
-|KEY_LEFT_CTRL  |0x80 |128
-|KEY_LEFT_SHIFT |0x81 |129
-|KEY_LEFT_ALT   |0x82 |130
-|KEY_LEFT_GUI   |0x83 |131
-|KEY_RIGHT_CTRL |0x84 |132
-|KEY_RIGHT_SHIFT |0x85 |133
-|KEY_RIGHT_ALT  |0x86 |134
-|KEY_RIGHT_GUI  |0x87 |135
+|Key	|Hexadecimal value	|Decimal value	|Notes
+
+|KEY_LEFT_CTRL  |0x80 |128 |
+|KEY_LEFT_SHIFT |0x81 |129 |
+|KEY_LEFT_ALT   |0x82 |130 |option (⌥) on Mac
+|KEY_LEFT_GUI   |0x83 |131 |OS logo, command (⌘) on Mac
+|KEY_RIGHT_CTRL |0x84 |132 |
+|KEY_RIGHT_SHIFT |0x85 |133 |
+|KEY_RIGHT_ALT  |0x86 |134 |also AltGr, option (⌥) on Mac
+|KEY_RIGHT_GUI  |0x87 |135 |OS logo, command (⌘) on Mac
 |===
 
 [float]