Clarify the fence description

The fence description uses the phrase "a matching pair of hyphens",
which can be misread as describing exactly two individual hyphens.
The constraints on fence length and matching are also compressed into
a single sentence with a trailing subclause ("from 3 to 255") that
reads as nonrestrictive.

Let's give each constraint its own sentence: what a fence is, where
it must appear, the length bounds on the opening fence, the matching
requirement for the closing fence, and trailing whitespace.  This
makes the structure clearer.

Author: traviscross

src/frontmatter.md

@@ -54,7 +54,7 @@ r[frontmatter.document]
 Frontmatter may only be preceded by a [shebang] and [whitespace].
 
 r[frontmatter.fence]
-The delimiters are referred to as a *fence*. The opening and closing fences must be at the start of a line. They must be a matching pair of hyphens (`-`), from 3 to 255. A fence may be followed by horizontal whitespace.
+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 of either fence may be followed by horizontal whitespace.
 
 r[frontmatter.infostring]
 Following the opening fence may be an infostring for identifying the intention of the contained content. An infostring may be followed by horizontal whitespace.