New material on type captures; work on introduction and parameter separators; some fixes here and there (#4669)

* Complete overhaul of section on double semicolon (;;) parameter separator in file Language/signatures.rakudoc
* Expand section Type captures (adding text on coercion and constraints). Also various fixes.
* correct definition of subset Even, and adjust output
* clarify explanation re. Nil/is-default
* further clarify difference between parameters and variables with default values when assigned Nil
* restore text that apparently was unintentionally deleted in 2024 with commit 348e623
* fix sentence relationship in section "Constraining signatures of Callables", and make examples more pertinent
* skip test for failing code & align code
* sigilled -> sigiled
* add missing formatting code
* fix Num -> Numeric
* captured types can in fact be used as return types. +make spaces consistent
* fix rakudoc syntax (C<->> to C«->») and add note to pod.rakudoc
* add example for using captured type as return type constraint
* add example combining capture with type smiley
* add resulting signatures to examples
* add link to explanation of itemization
* Show how itemization prevents flattening in slurpies. Move section 'Type captures' up, and sections on slurpies down.
* Moving section 'Type captures' up keeps the material on type constraints together.
* Moving sections on slurpies down so that the more elementary section 'Positional vs. named arguments' appears first.
* consistent usage of puntuation before examples
* Add basic slurpies to introduction

Author: schultzdavid

doc/Language/perl-op.rakudoc

@@ -52,7 +52,7 @@ also used for method calls.  So Perl's C«$arrayref->[7]» becomes
 Raku's C<$arrayref.[7]> and C«$user->name» becomes C<$user.name>.
 Note that dereferencing is rare in Raku.
 
-Raku uses the C<->> to attach L<C<Signature>|/type/Signature>s to
+Raku uses the C«->» to attach L<C<Signature>|/type/Signature>s to
 L<C<Block>|/type/Block>s.
 
 =head2 Auto-increment and auto-decrement

doc/Language/pod.rakudoc

@@ -550,9 +550,16 @@ C<my $var = 1; say $var;>
 
 C<my $var = 1; say $var;>
 
+If the enclosed code itself contains an unmatched C«>»,
+enclose it in C<C« »>
+=for code :lang<rakudoc>
+C«sub f(Int --> Int) {}»
+
+C«sub f(Int --> Int) {}»
+
 =head2 Links
 
-To create a link enclose it in C<L< >>:
+To create a link enclose it in C<L< >>
 
 =for code :lang<rakudoc>
 Raku homepage L<https://raku.org>