[DOC] Tweaks for String#chr

doc/string/chr.rdoc

@@ -0,0 +1,8 @@
+Returns a string containing the first character of +self+:
+
+  'hello'.chr     # => "h"
+  'тест'.chr      # => "т"
+  'こんにちは'.chr # => "こ"
+  ''.chr          # => ""
+
+Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].

string.c

@@ -6689,10 +6689,7 @@ rb_str_clear(VALUE str)
  *  call-seq:
  *    chr -> string
  *
- *  Returns a string containing the first character of +self+:
+ *  :include: doc/string/chr.rdoc
- *
- *    s = 'foo' # => "foo"
- *    s.chr     # => "f"
  *
  */