Adds headings to the return type arrow, closes #2921

doc/Language/regexes.pod6

@@ -901,11 +901,9 @@ character of an input string.
 
 =head1 Zero-width assertions
 
-Zero-Width Assertions can help you implement your own anchor.
-
-A zero-width assertion turns another regex into an anchor, making
-them consume no characters of the input string. There are two variants:
-lookahead and lookbehind assertions.
+Zero-Width assertions can help you implement your own anchor: it turns another
+regex into an anchor, making them consume no characters of the input string.
+There are two variants: lookahead and lookbehind assertions.
 
 Technically, anchors are also zero-width assertions, and they can look
 both ahead and behind.

doc/Type/Signature.pod6

@@ -465,11 +465,12 @@ chain.
 Type captures are not supported.
 
 X«|-->»
-=head4 C<-->>
+X«|Return type arrow»
+=head4 Return type arrow: C<-->>
 
-This form is preferred for several reasons:
-(1) it can handle constant values while the others can't;
-(2) for consistency, it is the only form accepted on this site;
+This form of indicating return types (or constants) in the signature is
+preferred, since it can handle constant values while the others can't. For
+consistency, it is the only form accepted on this site.
 
 The return type arrow has to be placed at the end of the parameter list,
 with or without a C<,> before it.