[pango] Expand docs for pango_font_get_hb_font

[Matthias Clasen <matthiasc src gnome org>]

commit 5c74e9e573d9af5995c1e1aa44538561788dcdc3
Author: Matthias Clasen <mclasen redhat com>
Date:   Thu Jul 18 16:25:01 2019 -0700

    Expand docs for pango_font_get_hb_font

 pango/fonts.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/pango/fonts.c b/pango/fonts.c
index 262ed15f..10e5463d 100644
--- a/pango/fonts.c
+++ b/pango/fonts.c
@@ -1862,6 +1862,10 @@ pango_font_get_font_map (PangoFont *font)
  *
  * Get a hb_font_t object backing this font.
  *
+ * Note that the objects returned by this function
+ * are cached and immutable. If you need to make
+ * changes to the hb_font_t, use hb_font_create_sub_font().
+ *
  * Returns: (transfer none) (nullable): the hb_font_t object backing the
  *          font, or %NULL if the font does not have one
  *