Improve consistency in UNICODEMAP code and docs, update docs/understanding_qmk (#4774)

* Remove unused UNICODE(n) macro, update docs

* Add note about max length of unicode_map to docs

* QK_UNICODE_MAP → QK_UNICODEMAP

* Refactor process_unicode_map

* process_unicode_map → process_unicodemap

This is done for consistency: UNICODEMAP is the method (hence UNICODEMAP_ENABLE, process_unicodemap), whereas unicode_map is the mapping table itself.

* Update references and ordering in docs/understanding_qmk

* Add additional note to docs/understanding_qmk

* &unicode_map[index] → unicode_map + index

This avoids the issue of the compiler sometimes complaining about the array index being out of range

* Update docs/getting_started_make_guide

* Update method sections in docs/feature_unicode

1 files changed, 1 insertions, 1 deletions

diff --git a/quantum/process_keycode/process_unicode_common.c b/quantum/process_keycode/process_unicode_common.c
index 3286f45b5..b64feb700 100644
--- a/quantum/process_keycode/process_unicode_common.c
+++ b/quantum/process_keycode/process_unicode_common.c
@@ -216,7 +216,7 @@ bool process_unicode_common(uint16_t keycode, keyrecord_t *record) {
 #if   defined(UNICODE_ENABLE)
   return process_unicode(keycode, record);
 #elif defined(UNICODEMAP_ENABLE)
-  return process_unicode_map(keycode, record);
+  return process_unicodemap(keycode, record);
 #elif defined(UCIS_ENABLE)
   return process_ucis(keycode, record);
 #else