gio: Add various missing property documentation comments

Previously these properties would have been documented using the strings
from the pspec, but those will be removed in the following commit. Re-add
the documentation using those strings, but as gi-docgen documentation
comments.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #2991

gio/gbufferedinputstream.c

@@ -157,6 +157,11 @@ g_buffered_input_stream_class_init (GBufferedInputStreamClass *klass)
   bstream_class->fill_async = g_buffered_input_stream_real_fill_async;
   bstream_class->fill_finish = g_buffered_input_stream_real_fill_finish;
 
+  /**
+   * GBufferedInputStream:buffer-size:
+   *
+   * The size of the backend buffer, in bytes.
+   */
   g_object_class_install_property (object_class,
                                    PROP_BUFSIZE,
                                    g_param_spec_uint ("buffer-size",