Add basic docs for the Signal enum

Closes #2867

Author: coke

doc/Type/Signal.rakudoc

@@ -0,0 +1,17 @@
+=begin pod :kind("Type") :subkind("enum") :category("domain-specific")
+
+=TITLE enum Signal
+
+=SUBTITLE List of supported signals
+
+    enum Signal ()
+
+A dynamically generated L<enum|/language/enumeration> containing numeric/named values for
+supported signals.  For example:
+
+    say Signal.keys.sort[^3] # show the first 3 alphabetically
+    # OUTPUT: «(SIGABRT SIGALRM SIGBREAK)»
+
+See L<related routines|/routine/signal>.
+
+=end pod

xt/pws/code.pws

@@ -440,6 +440,9 @@ setencoding
 setgoal
 sev
 shoriyuru
+sigabrt
+sigalrm
+sigbreak
 skippingarray
 sl
 sleepsorter