Fix named-grid-area-no-invalid README (#5187)

* Fix named-grid-area-no-invalid README

* Update lib/rules/named-grid-areas-no-invalid/README.md

Co-authored-by: Masafumi Koba <473530+ybiquitous@users.noreply.github.com>

Co-authored-by: Masafumi Koba <473530+ybiquitous@users.noreply.github.com>

lib/rules/named-grid-areas-no-invalid/README.md

@@ -4,56 +4,47 @@ Disallow invalid named grid areas.
 
 <!-- prettier-ignore -->
 ```css
-a { grid-template-areas: 
+a { grid-template-areas:
       "a a a"
       "b b b"; }
 /**   ↑
  *  This named grid area */
 ```
 
+For a named grid area to be valid, all strings must define:
+
+- the same number of cell tokens
+- at least one cell token
+
+And all named grid areas that spans multiple grid cells must form a single filled-in rectangle.
+
 ## Options
 
 ### `true`
 
 The following patterns are considered violations:
 
-All strings must define the same number of cell tokens.
-
 <!-- prettier-ignore -->
 ```css
-a { grid-template-areas: "a a a"
-                         "b b b b"; }
+a { grid-template-areas: "" }
 ```
 
-All strings must define at least one cell token.
-
 <!-- prettier-ignore -->
 ```css
-a { grid-template-areas: "" }
+a { grid-template-areas: "a a a"
+                         "b b b b"; }
 ```
 
-All named grid areas that spans multiple grid cells must form a single filled-in rectangle.
-
 <!-- prettier-ignore -->
 ```css
 a { grid-template-areas: "a a a"
                          "b b a"; }
 ```
 
-The following patterns are _not_ considered violations:
+The following pattern is _not_ considered a violation:
 
 <!-- prettier-ignore -->
 ```css
 a { grid-template-areas: "a a a"
                          "b b b"; }
 ```
-
-<!-- prettier-ignore -->
-```css
-a { grid-template-areas: "a a a" "b b b"; }
-```
-
-<!-- prettier-ignore -->
-```css
-a { grid-template-areas: none; }
-```