Flesh out the frontmatter removal section

The frontmatter removal section in `input-format.md` is a single
sentence ("After some whitespace, frontmatter may next appear in
the input") that doesn't clearly describe the removal behavior.  The
"some" is ambiguous (does it mean "a quantity of" or "certain"?),
and "may next appear" is slightly awkward.  By contrast, the shebang
removal section provides a full description with an example.

Let's rewrite the section with a precise description of the removal
process and add an annotated example.

Author: traviscross

src/input-format.md

@@ -72,7 +72,20 @@ The shebang is removed from the input sequence (and is therefore ignored).
 r[input.frontmatter]
 ## Frontmatter removal
 
-After some [whitespace], [frontmatter] may next appear in the input.
+If the remaining input begins with a [frontmatter] fence, optionally preceded by lines containing only [whitespace], the [frontmatter] and any preceding whitespace are removed.
+
+For example, given the following file:
+
+<!-- ignore: test runner doesn't support frontmatter -->
+```rust,ignore
+--- cargo
+package.edition = 2024
+---
+
+fn main() {}
+```
+
+The first three lines (the opening fence, body, and closing fence) would be removed, leaving an empty line followed by `fn main() {}`.
 
 r[input.tokenization]
 ## Tokenization