Touch uniprop documentation

This was an attempt to fix #1045, but I realized that uniprop-int and
uniprop-str don't return anything sensible¹ at this moment.

¹ – https://irclog.perlgeek.de/perl6/2018-05-06#i_16134827

Author: AlexDaniel

doc/Type/Cool.pod6

@@ -909,14 +909,13 @@ C<$pattern> is applied to the remaining characters of C<$string>.
 
 Defined as:
 
-    multi sub uniprop(Str(Cool), |c)
+    multi sub uniprop(Str:D, |c)
+    multi sub uniprop(Int:D $code)
     multi sub uniprop(Int:D $code, Stringy:D $propname)
-    multi sub uniprop(Str, $code, Stringy:D $propname)
     multi method uniprop(|c)
 
-Interprets the invocant as a L<Str|/type/Str>, and returns the
-L<unicode property|http://userguide.icu-project.org/strings/properties> of the first
-character. If no property is specified returns the
+Returns the L<unicode property|http://userguide.icu-project.org/strings/properties>
+of the first character. If no property is specified returns the
 L<General Category|https://en.wikipedia.org/wiki/Unicode_character_property#General_Category>.
 Returns a Bool for Boolean properties.