Clarify the body restrictions

The body restriction sentence combines two unrelated constraints --
the hyphen-line restriction and the carriage-return ban -- in a single
sentence joined by "or".  This makes "or carriage returns" read as
parallel to "hyphens", as though the line could maybe start with
carriage returns.

Let's split these into two separate sentences so that each constraint
stands on its own.

Author: traviscross

src/frontmatter.md

@@ -60,7 +60,8 @@ r[frontmatter.infostring]
 The opening fence, after optional horizontal whitespace, may be followed by an infostring that identifies the format or purpose of the body. An infostring may be followed by horizontal whitespace.
 
 r[frontmatter.body]
-The body of the frontmatter may contain any content except for a line starting with as many or more hyphens (`-`) than in the fences or carriage returns.
+No line in the body may start with a sequence of hyphens (`-`) equal to or longer than the opening fence. The body may not contain any carriage returns (that survive [CRLF normalization]).
 
+[CRLF normalization]: input.crlf
 [shebang]: input-format.md#shebang-removal
 [whitespace]: whitespace.md