Update KC_GESC documentation (qmk#5053)

* Update KC_GESC documentation

 - document caveats on OSX and expected behavior with KC_GESC and GUI
 - clarify documentation slightly
 - give examples of expected output in actual use

* Update OSX -> macOS

* Captilization

* remove simply

docs/feature_grave_esc.md

@@ -4,14 +4,22 @@ If you're using a 60% keyboard, or any other layout with no F-row, you will have
 
 ## Usage
 
-Replace the `KC_GRAVE` key in your keymap (usually to the left of the `1` key) with `KC_GESC`. When pressed it will behave like `KC_ESC`, but with Shift or GUI held it will send `KC_GRAVE`.
+Replace the `KC_GRAVE` key in your keymap (usually to the left of the `1` key) with `KC_GESC`. Most of the time this key will output `KC_ESC` when pressed. However, when Shift or GUI are held down it will output `KC_GRV` instead.
+
+## What Your OS Sees
+
+If Mary presses GESC on her keyboard, the OS will see an KC_ESC character. Now if Mary holds Shift down and presses GESC it will output `~`, or a shifted backtick. Now if she holds GUI/CMD/WIN, it will output a simple <code>&#96;</code> character.
 
 ## Keycodes
 
 |Key      |Aliases    |Description                                                       |
 |---------|-----------|------------------------------------------------------------------|
 |`KC_GESC`|`GRAVE_ESC`|Escape when pressed, <code>&#96;</code> when Shift or GUI are held|
 
+### Caveats
+
+ * On macOS CMD/GUI + KC_GRV is actually mapped to a hot key so it will not output a backtick.
+
 ## Configuration
 
 There are several possible key combinations this will break, among them Control+Shift+Escape on Windows and Command+Option+Escape on macOS. To work around this, you can `#define` these options in your `config.h`: