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, 7 insertions, 9 deletions

diff --git a/docs/getting_started_make_guide.md b/docs/getting_started_make_guide.md
index e515411901..adc1aed751 100644
--- a/docs/getting_started_make_guide.md
+++ b/docs/getting_started_make_guide.md
@@ -93,19 +93,17 @@ This enables MIDI sending and receiving with your keyboard. To enter MIDI send m
 
 `UNICODE_ENABLE`
 
-This allows you to send unicode symbols via `UC(<unicode>)` in your keymap. Only codes up to 0x7FFF are currently supported.
+This allows you to send Unicode characters using `UC(<code point>)` in your keymap. Code points up to `0x7FFF` are supported. This covers characters for most modern languages, as well as symbols, but it doesn't cover emoji.
 
 `UNICODEMAP_ENABLE`
 
-This allows sending unicode symbols using `X(<unicode>)` in your keymap. Codes
-up to 0xFFFFFFFF are supported, including emojis. You will need to maintain
-a separate mapping table in your keymap file.
+This allows you to send Unicode characters using `X(<map index>)` in your keymap. You will need to maintain a mapping table in your keymap file. All possible code points (up to `0x10FFFF`) are supported. 
 
-Known limitations:
-- Under Mac OS, only codes up to 0xFFFF are supported.
-- Under Linux ibus, only codes up to 0xFFFFF are supported (but anything important is still under this limit for now).
+`UCIS_ENABLE`
 
-Characters out of range supported by the OS will be ignored.
+This allows you to send Unicode characters by inputting a mnemonic corresponding to the character you want to send. You will need to maintain a mapping table in your keymap file. All possible code points (up to `0x10FFFF`) are supported.
+
+For further details, as well as limitations, see the [Unicode page](feature_unicode.md).
 
 `BLUETOOTH_ENABLE`
 
@@ -117,7 +115,7 @@ This allows you output audio on the C6 pin (needs abstracting). See the [audio p
 
 `FAUXCLICKY_ENABLE`
 
-Uses buzzer to emulate clicky switches. A cheap imitation of the Cherry blue switches. By default, uses the C6 pin, same as AUDIO_ENABLE.
+Uses buzzer to emulate clicky switches. A cheap imitation of the Cherry blue switches. By default, uses the C6 pin, same as `AUDIO_ENABLE`.
 
 `VARIABLE_TRACE`