keys: Rename the help category of message compose box actions.

The category name in the code: 'msg_compose' -> 'compose_box'
The user visible name: 'Composing a message' -> 'Writing a message'

This is to emphasize its difference from the other 2 newly created
categories using the title prefix 'Compose', to both users and
developers.

Hotkeys document regenerated.

docs/hotkeys.md

@@ -82,7 +82,7 @@
 |Reply quoting the current message text|<kbd>></kbd>|
 |Reply directly to the sender of the current message|<kbd>R</kbd>|
 
-## Composing a Message
+## Writing a message
 |Command|Key Combination|
 | :--- | :---: |
 |Cycle through recipient and content boxes|<kbd>Tab</kbd>|

zulipterminal/config/keys.py

@@ -129,28 +129,28 @@ class KeyBinding(TypedDict):
     'CYCLE_COMPOSE_FOCUS': {
         'keys': ['tab'],
         'help_text': 'Cycle through recipient and content boxes',
-        'key_category': 'msg_compose',
+        'key_category': 'compose_box',
     },
     'SEND_MESSAGE': {
         'keys': ['ctrl d', 'meta enter'],
         'help_text': 'Send a message',
-        'key_category': 'msg_compose',
+        'key_category': 'compose_box',
     },
     'SAVE_AS_DRAFT': {
         'keys': ['meta s'],
         'help_text': 'Save current message as a draft',
-        'key_category': 'msg_compose',
+        'key_category': 'compose_box',
     },
     'AUTOCOMPLETE': {
         'keys': ['ctrl f'],
         'help_text': ('Autocomplete @mentions, #stream_names, :emoji:'
                       ' and topics'),
-        'key_category': 'msg_compose',
+        'key_category': 'compose_box',
     },
     'AUTOCOMPLETE_REVERSE': {
         'keys': ['ctrl r'],
         'help_text': 'Cycle through autocomplete suggestions in reverse',
-        'key_category': 'msg_compose',
+        'key_category': 'compose_box',
     },
     'ADD_REACTION': {
         'keys': [':'],
@@ -170,7 +170,7 @@ class KeyBinding(TypedDict):
     'NARROW_MESSAGE_RECIPIENT': {
         'keys': ['meta .'],
         'help_text': 'Narrow to compose box message recipient',
-        'key_category': 'msg_compose',
+        'key_category': 'compose_box',
     },
     'TOGGLE_NARROW': {
         'keys': ['z'],
@@ -421,7 +421,7 @@ class KeyBinding(TypedDict):
     "stream_list": "Stream list actions",
     "new_msg": "Compose: New Message",
     "reply": "Compose: Replies",
-    "msg_compose": "Composing a Message",
+    "compose_box": "Writing a message",
     "editor_navigation": "Editor: Navigation",
     "editor_text_manipulation": "Editor: Text Manipulation",
 }