Clarify frontmatter position rule

The `r[frontmatter.document]` rule said "Frontmatter may only be
preceded by a shebang and whitespace", where the "and" could be
misread as requiring both a shebang and whitespace rather than listing
the set of things allowed to precede frontmatter.

Since we merged the shebang prose revision (#2192),
the shebang position rule now reads as a positive statement of where
the shebang may appear.  Let's follow the same pattern here: state
positively where frontmatter may appear rather than leaning on "only"
and a negative constraint.

In this commit, we rewrite the rule as "Frontmatter may appear at
the start of the file or after a shebang, in either case optionally
preceded by whitespace."  This resolves the ambiguity and parallels
the shebang position rule's structure.

Author: traviscross

src/frontmatter.md

@@ -51,7 +51,7 @@ fn main() {
 ```
 
 r[frontmatter.document]
-Frontmatter may only be preceded by a [shebang] and [whitespace].
+Frontmatter may appear at the start of the file or after a [shebang], in either case optionally preceded by [whitespace].
 
 r[frontmatter.fence]
 Frontmatter must start and end with a *fence*. Each fence must start at the beginning of a line. The opening fence must consist of at least 3 and no more than 255 hyphens (`-`). The closing fence must have exactly the same number of hyphens as the opening fence. The hyphens may be followed by [horizontal whitespace].