Fixes pid definition, closes #2821

doc/Type/Proc.pod6

@@ -24,8 +24,9 @@ my $p1 = run 'echo', 'Hello, world', :out;
 my $p2 = run 'cat', '-n', :in($p1.out), :out;
 say $p2.out.get;
 
-You can also feed the C<:in> pipe directly from your program, by setting it
-to C<True>, which will make the pipe available via C<.in> method on the C<Proc>:
+You can also feed the C<:in> (standard input) pipe directly from your program,
+by setting it to C<True>, which will make the pipe available via C<.in> method
+on the C<Proc>:
 
 =for code
 my $p = run "cat", "-n", :in, :out;
@@ -35,7 +36,7 @@ say $p.out.slurp: :close;
 # OUTPUT: «1  Hello,␤
 #          2  world!␤»
 
-In order to capture the standard error C<:err> can be supplied:
+In order to capture the standard error, C<:err> can be supplied:
 
 =for code
 my $p = run "ls", "-l", ".", "qqrq", :out, :err;
@@ -179,9 +180,9 @@ C<shell> or C<run>.
 
 =head2 method pid
 
-    method pid(Proc:D:)
+    method pid()
 
-Returns the C<$*PID> value of the process if available, or C<Nil>.
+Returns the C<PID> value of the process if available, or C<Nil>.
 
 =head2 method exitcode