Update Signal.signame doc [ci-skip]

* signal.c (sig_signame): [DOC] Add documentation in the case of
  return nil.  [Fix GH-1449]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56280 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -1,3 +1,8 @@
+Wed Sep 28 13:00:25 2016  ksss  <co000ri@gmail.com>
+
+	* signal.c (sig_signame): [DOC] Add documentation in the case of
+	  return nil.  [Fix GH-1449]
+
 Wed Sep 28 10:43:27 2016  URABE Shyouhei  <shyouhei@ruby-lang.org>
 
 	* .travis.yml: delete comments. They were outdated.

signal.c

@@ -238,9 +238,10 @@ signo2signm(int no)
 
 /*
  * call-seq:
- *     Signal.signame(signo)  ->  string
+ *     Signal.signame(signo)  ->  string or nil
  *
- *  convert signal number to signal name
+ *  Convert signal number to signal name.
+ *  Returns +nil+ if the signo is an invalid signal number.
  *
  *     Signal.trap("INT") { |signo| puts Signal.signame(signo) }
  *     Process.kill("INT", 0)