Rewrite frontmatter intro sentence

The intro under `frontmatter.intro` said "an optional section for
content intended for external tools without requiring these tools
to have full knowledge of the Rust grammar."  This was a negative
construction (what frontmatter doesn't require) rather than a positive
one (what it is and what it enables).

In this commit, we rewrite the intro as "an optional section of
metadata whose syntax allows external tools to read it without parsing
Rust."  This tells the reader three things in one sentence: what
frontmatter is, who it's for, and the key design property.

Author: traviscross

src/frontmatter.md

@@ -35,7 +35,7 @@ INFOSTRING -> (XID_Start | `_`) ( XID_Continue | `-` | `.` )*
 ```
 
 r[frontmatter.intro]
-Frontmatter is an optional section for content intended for external tools without requiring these tools to have full knowledge of the Rust grammar.
+Frontmatter is an optional section of metadata whose syntax allows external tools to read it without parsing Rust.
 
 > [!EXAMPLE]
 > <!-- ignore: test runner doesn't support frontmatter -->