Correct and clarify some parts of ruby-to-perl6 nutshell.

Also fixed the Dockerfile so that make docker-htmlify works.
Specifically, I made it no longer purge the buildDeps because when
npm install in highlights/, it needs build-essentials to compile some
native module.

Author: Jack Kuan

Dockerfile

@@ -25,9 +25,7 @@ RUN buildDeps='         \
     && n=/usr/local/bin/n \
     && curl -fsSL https://raw.githubusercontent.com/tj/n/master/bin/n > "$n" \
     && chmod +x "$n"      \
-    && n stable           \
-                          \
-    && apt-get purge --yes --auto-remove $buildDeps
+    && n stable
 
 WORKDIR /perl6/doc
 EXPOSE  3000

doc/Language/rb-nutshell.pod6

@@ -311,7 +311,7 @@ sub f(&g:($)) {
 f(-> $n { say "Hi {$n}" }); # Explicit argument
 f -> $n { say "Hi {$n}" };  # Explicit argument, no parenthesis
 
-# Additionally, if 'f' is a method on instance 'obj' you can use C<:>
+# Additionally, if 'f' is a method on instance 'obj' you can use ':'
 # instead of parenthesis
 my $obj; ...;
 $obj.f(-> $n { say "Hi {$n}" });  # Explicit argument
@@ -321,15 +321,28 @@ $obj.f: -> $n { say "Hi {$n}" };  # Explicit argument, no parenthesis
 =head3 C<*> Slurpy params / argument expansion
 
 In Ruby you can declare an argument to slurp the remainder of the passed
-parameters into an array using a C<*> prefix. It works the same way in Perl 6:
+parameters into an array using a C<*> prefix. It works similarly in Perl 6:
 
 =for code :lang<ruby>
 def foo(*args); puts "I got #{args.length} args!"; end # Ruby
 =for code
 sub foo(*@args) { say "I got #{@args.elems} args!" }   # Perl 6
 
+The Perl 6 version above is slightly different in that when it slurps in the
+arguments into @args, one level of nesting, if any, is automatically removed:
+
+=for code
+sub foo(*@args) { dd @args }
+foo([1, [2, 3], 4], 5, [6, 7]);   # [1, [2, 3], 4, 5, 6, 7]
+
+To preserve the structure of the arguments, you can use C<**>:
+
+=for code
+sub foo(**@args) { dd @args }
+foo([1, [2, 3], 4], 5, [6, 7]);  # [[1, [2, 3], 4], 5, [6, 7]]
+
 You might want to expand an array into a set of arguments. In Perl 6 this is
-also done using C<*>:
+done using a L<Slip|/type/Slip> C<|>:
 
 =for code :lang<ruby>
 args = %w(a b c)         # Ruby
@@ -338,7 +351,7 @@ foo(*args)
 =for code
 sub foo($q, $r, $s) { ... };
 my @args = <a b c>;       # Perl 6
-foo(@*args);
+foo(|@args);
 
 Perl 6 has many more advanced ways of passing parameters and receiving
 arguments, see L<Signatures|/language/functions#Signatures> and
@@ -355,7 +368,7 @@ $!foo    # Private instance variable
 $.foo    # Instance variable accessor
 $*foo    # Dynamically scoped variable
 $^foo    # A positional (placeholder) parameter to a block
-$:foo    # A named parameter
+$:foo    # A named (placeholder) parameter to a block
 $=foo    # POD (documentation) variables
 $?FILE   # Current source filename. The ? twigil indicates a compile-time value
 $~foo    # Sublanguage seen by parser, uncommon