Includes new form of contains

Also clarifies haystack metaphor. @lizmat changed this very recently,
it's likely that it's not really done that way in the current version,
but it matches roast, and over all, it closes #2334. Thanks!

doc/Type/Str.pod6

@@ -45,19 +45,19 @@ Examples:
 
 Defined as:
 
-    multi method contains(Str:D: Cool:D $needle)
-    multi method contains(Str:D: Str:D $needle)
-    multi method contains(Str:D: Cool:D $needle, Int(Cool:D) $pos)
-    multi method contains(Str:D: Str:D $needle, Int:D $pos)
-
-Coerces the invocant (represented in the signature by C<Str:D:> and
-first argument (which we are calling C<$needle>) to L<C<Str>|/type/Str>
-(if it's not already, that is, the first and third C<multi>), and
-searches for C<$needle> in the string starting from C<$pos> characters
-into the string. Returns C<True> if C<$needle> is found. C<$pos> is an
-optional parameter, and if it's not present, C<contains> will search
-from the beginning of the string (using the first two forms of the
-C<multi>).
+    multi method contains(Str:D: Cool:D $needle --> Bool:D)
+    multi method contains(Str:D: Str:D $needle --> Bool:D)
+    multi method contains(Str:D: Cool:D $needle, Int(Cool:D) $pos --> Bool:D)
+    multi method contains(Str:D: Str:D $needle, Int:D $pos --> Bool:D)
+
+Coerces the invocant (represented in the signature by C<Str:D:>, that would be
+the I<haystack>) and first argument (which we are calling C<$needle>) to
+L<C<Str>|/type/Str> (if it's not already, that is, in the first and third
+C<multi> forms), and searches for C<$needle> in the invocant (or I<haystack>)
+starting from C<$pos> characters into the string, if that is included as an
+argument. Returns C<True> if C<$needle> is found. C<$pos> is an optional
+parameter, and if it's not present, C<contains> will search from the beginning
+of the string (using the first two forms of the C<multi>).
 
     say <Hello, World>.contains('Hello', 0);   # OUTPUT: «True␤»
     say "Hello, World".contains('Hello');      # OUTPUT: «True␤»
@@ -1434,4 +1434,4 @@ C<Numeric>.
 
 =end pod
 
-# vim: expandtab softtabstop=4 shiftwidth=4 ft=perl6
+# vim: expandtab softtabstop=4 shiftwidth=4 ft=perl6