[ruby/stringio] [DOC] Doc for StringIO#getbyte

(https://github.com/ruby/stringio/pull/162)

https://github.com/ruby/stringio/commit/95a7dd592c

doc/stringio/getbyte.rdoc

@@ -0,0 +1,29 @@
+Reads and returns the next integer byte (not character) from the stream:
+
+  s = 'foo'
+  s.bytes       # => [102, 111, 111]
+  strio = StringIO.new(s)
+  strio.getbyte # => 102
+  strio.getbyte # => 111
+  strio.getbyte # => 111
+
+Returns +nil+ if at end-of-stream:
+
+  strio.eof?    # => true
+  strio.getbyte # => nil
+
+Returns a byte, not a character:
+
+  s = 'тест'
+  s.bytes       # => [209, 130, 208, 181, 209, 129, 209, 130]
+  strio = StringIO.new(s)
+  strio.getbyte # => 209
+  strio.getbyte # => 130
+
+  s = 'こんにちは'
+  s.bytes       # => [227, 129, 147, 227, 130, 147, 227, 129, 171, 227, 129, 161, 227, 129, 175]
+  strio = StringIO.new(s)
+  strio.getbyte # => 227
+  strio.getbyte # => 129
+
+Related: StringIO.getc.

ext/stringio/stringio.c

@@ -990,10 +990,10 @@ strio_getc(VALUE self)
 
 /*
  * call-seq:
- *   getbyte -> byte or nil
+ *   getbyte -> integer or nil
+ *
+ * :include: stringio/getbyte.rdoc
  *
- * Reads and returns the next 8-bit byte from the stream;
- * see {Byte IO}[rdoc-ref:IO@Byte+IO].
  */
 static VALUE
 strio_getbyte(VALUE self)