docs: clarify subject-case [resolves conventional-changelog#3638] (conventional-changelog#3640)

* docs(subject-case): standard 'condition' language

* docs(subject-case): standard lower-case example

Author: olets

docs/reference-rules.md

@@ -278,7 +278,7 @@ Infinity
 
 #### subject-case
 
-- **condition**: `subject` is in one of the cases `['sentence-case', 'start-case', 'pascal-case', 'upper-case']`
+- **condition**: `subject` is in case `value`
 - **rule**: `always`
 - **value**
 
@@ -290,7 +290,7 @@ Infinity
 
 ```js
 [
-  'lower-case',    // default
+  'lower-case',    // lower case
   'upper-case',    // UPPERCASE
   'camel-case',    // camelCase
   'kebab-case',    // kebab-case