Add Post Processing to process_record (#4892)

* Improve process_record system

Code based on @colinta's

* Rename and better handle functions

* Fix incorrect function call to process_record_user

* Add documentation for post_process_record

* Add both get_event_keycode and get_record_keycode functions

And add some comments about these functions

* Update code format

* Cleanup merge artifacts

docs/feature_macros.md

@@ -88,6 +88,46 @@ const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
 };
 ```
 
+### Advanced Macros
+
+In addition to the `process_record_user()` function, is the `post_process_record_user()` function. This runs after `process_record` and can be used to do things after a keystroke has been sent.  This is useful if you want to have a key pressed before and released after a normal key, for instance. 
+
+In this example, we modify most normal keypresses so that `F22` is pressed before the keystroke is normally sent, and release it __only after__ it's been released.
+
+```c
+static uint8_t f22_tracker;
+
+bool process_record_user(uint16_t keycode, keyrecord_t *record) {
+  switch (keycode) {
+    case KC_A ... KC_F21: //notice how it skips over F22
+    case KC_F23 ... KC_EXSEL: //exsel is the last one before the modifier keys
+      if (record->event.pressed) {
+        register_code(KC_F22); //this means to send F22 down
+        f22_tracker++;
+        register_code(keycode);
+        return false;
+      }
+      break;
+  }
+  return true;
+}
+
+void post_process_record_user(uint16_t keycode, keyrecord_t *record) {
+  switch (keycode) {
+    case KC_A ... KC_F21: //notice how it skips over F22
+    case KC_F23 ... KC_EXSEL: //exsel is the last one before the modifier keys
+      if (!record->event.pressed) {
+        f22_tracker--;
+        if (!f22_tracker) {
+            unregister_code(KC_F22); //this means to send F22 up
+        }
+      }
+      break;
+  }
+}
+```
+
+
 ### TAP, DOWN and UP
 
 You may want to use keys in your macros that you can't write down, such as `Ctrl` or `Home`.

docs/understanding_qmk.md

@@ -162,6 +162,15 @@ The `process_record()` function itself is deceptively simple, but hidden within
 
 At any step during this chain of events a function (such as `process_record_kb()`) can `return false` to halt all further processing.
 
+After this is called, `post_process_record()` is called, which can be used to handle additional cleanup that needs to be run after the keycode is normally handled. 
+
+* [`void post_process_record(keyrecord_t *record)`]()
+  * [`void post_process_record_quantum(keyrecord_t *record)`]()
+    * [Map this record to a keycode]()
+    * [`void post_process_clicky(uint16_t keycode, keyrecord_t *record)`]()
+    * [`void post_process_record_kb(uint16_t keycode, keyrecord_t *record)`]()
+      * [`void post_process_record_user(uint16_t keycode, keyrecord_t *record)`]()
+      
 <!--
 #### Mouse Handling