[sysprof] capture: add doc for new_from_fd

[Christian Hergert <chergert src gnome org>]

commit 7f889b487da51cb052f947e9030a9067f811bdd3
Author: Christian Hergert <chergert redhat com>
Date:   Thu May 17 13:13:36 2018 +0100

    capture: add doc for new_from_fd

 lib/capture/sp-capture-reader.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)
---
diff --git a/lib/capture/sp-capture-reader.c b/lib/capture/sp-capture-reader.c
index df33174..5ea528d 100644
--- a/lib/capture/sp-capture-reader.c
+++ b/lib/capture/sp-capture-reader.c
@@ -106,6 +106,17 @@ sp_capture_reader_get_filename (SpCaptureReader *self)
   return self->filename;
 }
 
+/**
+ * sp_capture_reader_new_from_fd:
+ * @fd: an fd to take ownership from
+ * @error: a location for a #GError or %NULL
+ *
+ * Creates a new reader using the file-descriptor.
+ *
+ * This is useful if you don't necessarily have access to the filename itself.
+ *
+ * Returns: (transfer full): an #SpCaptureReader or %NULL upon failure.
+ */
 SpCaptureReader *
 sp_capture_reader_new_from_fd (int      fd,
                                GError **error)