doc: add explanation on the rule docs

Author: phanect

docs/rules/charset.md

@@ -3,8 +3,11 @@
 The corresponding EditorCongig property is `charset`.
 The backend ESLint rule is [`unicode-bom`](https://eslint.org/docs/rules/unicode-bom)
 
-This plugin works only when `utf-8` or `utf-8-bom` is specified. If other value is specified in .editorconfig, ESLint does not verify charset.
-ESLint only verifies if BOM is specified or not.
+Unlike other rules, this plugin works only when `utf-8` or `utf-8-bom` is specified. If other value is specified in .editorconfig, ESLint does not verify charset. ESLint only verifies if BOM is specified or not.
+
+If you set `charset = utf-8` in .editorconfig, the files **cannot** include a BOM.  
+If you set `charset = utf-8-bom` in .editorconfig, the files **must** include a BOM.  
+If you don't set `charset` in .editorconfig or set `charset = unset`, this rule is disabled.
 
 ## Options
 

docs/rules/eol-last.md

@@ -3,6 +3,10 @@
 The corresponding EditorCongig property is `insert_final_newline`.
 The backend ESLint rule is [`eol-last`](https://eslint.org/docs/rules/eol-last)
 
+If you set `insert_final_newline = true` in .editorconfig, the files **must** ends with a new line.  
+If you set `insert_final_newline = false` in .editorconfig, the files **must not** ends with a new line.  
+If you don't set `insert_final_newline` in .editorconfig or set `insert_final_newline = unset`, this rule is disabled.
+
 ## Options
 
 None

docs/rules/indent.md

@@ -3,6 +3,10 @@
 The corresponding EditorCongig property is `indent_style` and `indent_size`.
 The backend ESLint rule is [`indent`](https://eslint.org/docs/rules/indent) and [`@typescript-eslint/indent`](https://github.com/typescript-eslint/typescript-eslint/blob/master/packages/eslint-plugin/docs/rules/indent.md).
 
+If you set `indent_style = space` in .editorconfig, the indent size **must** be the value of `indent_size`.  
+If you set `indent_style = tab` in .editorconfig, the indent **must** be the tabs.  
+If you don't set `indent_style` in .editorconfig or set `indent_style = unset`, this rule is disabled.
+
 As documented, `@typescript-eslint/indent` is unstable currently. Please read typescript-eslint/typescript-eslint#1824 before using this rule for TypeScript.
 
 ## Options

docs/rules/linebreak-style.md

@@ -3,6 +3,10 @@
 The corresponding EditorCongig property is `end_of_line`.
 The backend ESLint rule is [`linebreak-style`](https://eslint.org/docs/rules/linebreak-style)
 
+If you set `end_of_line = lf` in .editorconfig, the linebreak code **must** be LF.  
+If you set `end_of_line = crlf` in .editorconfig, the linebreak code **must** be CRLF.  
+If you don't set `end_of_line` in .editorconfig or set `end_of_line = unset`, this rule is disabled.
+
 `end_of_line = cr` is not supported. When `end_of_line = cr` is specified in .editorconfig, This plugin does nothing.
 
 ## Options

docs/rules/no-trailing-spaces.md

@@ -3,6 +3,9 @@
 The corresponding EditorCongig property is `trim_trailing_whitespace`.
 The backend ESLint rule is [`no-trailing-spaces`](https://eslint.org/docs/rules/no-trailing-spaces)
 
+If you set `trim_trailing_whitespace = true` in .editorconfig, the trailing spaces **must** be removed.  
+If you set `trim_trailing_whitespace = false` or `trim_trailing_whitespace = unset`, or you don't set `trim_trailing_whitespace` in .editorconfig, this rule is disabled.
+
 ## Options
 
 > * `"skipBlankLines": false` (default) disallows trailing whitespace on empty lines