[gexiv2] doc: Add description of fallback to get_fnumber

[Jens Georg <jensgeorg src gnome org>]

commit 7507b01bf54512c34f6eae9b3c843881e9ddc49b
Author: Jens Georg <mail jensge org>
Date:   Fri Jan 5 12:52:14 2018 +0100

    doc: Add description of fallback to get_fnumber
    
    Add note about fall-back when f-number is not readily available

 gexiv2/gexiv2-metadata.h |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/gexiv2/gexiv2-metadata.h b/gexiv2/gexiv2-metadata.h
index 4a9a295..efa36a3 100644
--- a/gexiv2/gexiv2-metadata.h
+++ b/gexiv2/gexiv2-metadata.h
@@ -901,6 +901,8 @@ gboolean            gexiv2_metadata_get_exposure_time       (GExiv2Metadata *self, gint 
*nom, gi
  * @self: An instance of #GExiv2Metadata
  *
  * See <ulink url="https://en.wikipedia.org/wiki/F-number";></ulink> for more information.
+ * If Exiif.Photo.FNumber does not exist, it will fall back to calclating the FNumber from
+ * Exif.Photo.ApertureValue (if available);
  *
  * Returns: The exposure Fnumber as a gdouble, or -1.0 if tag is not present or invalid.
  */