clarify frontmatter title as level 1 heading

Author: xbinaryx

docs/rules/heading-increment.md

@@ -29,7 +29,7 @@ Goodbye World!
 
 The following options are available on this rule:
 
-* `frontmatterTitle: string` - A regex pattern to match title fields in front matter. The default pattern matches YAML (`title:`), TOML (`title =`), and JSON (`"title":`) formats. Set to an empty string to disable front matter title checking.
+* `frontmatterTitle: string` - A regex pattern to match title fields in front matter. Front matter titles matching this pattern are treated as level 1 headings. The default pattern matches YAML (`title:`), TOML (`title =`), and JSON (`"title":`) formats. Set to an empty string to disable front matter title checking.
 
 Examples of **incorrect** code for this rule: