Revert "Deprecate font kern API"

This reverts commit d219f899f4b2fb4b39ebc1dff9fb648fc5d6d112.

API change: Un-deprecate font kern API.

Part of https://github.com/harfbuzz/harfbuzz/issues/1682

We should document that this API is only necessary to hook up
non-OpenType kerning.  hb-ot-font will continue to NOT implement them.

docs/harfbuzz-sections.txt

diff --git a/docs/harfbuzz-sections.txt b/docs/harfbuzz-sections.txt
index f97902f..2f34b6a 100644
--- a/docs/harfbuzz-sections.txt
+++ b/docs/harfbuzz-sections.txt
@@ -195,14 +195,6 @@
 hb_unicode_decompose_compatibility_func_t
 hb_unicode_decompose_compatibility
 hb_unicode_funcs_set_decompose_compatibility_func
-hb_font_funcs_set_glyph_h_kerning_func
-hb_font_funcs_set_glyph_v_kerning_func
-hb_font_get_glyph_h_kerning
-hb_font_get_glyph_h_kerning_func_t
-hb_font_get_glyph_kerning_for_direction
-hb_font_get_glyph_kerning_func_t
-hb_font_get_glyph_v_kerning
-hb_font_get_glyph_v_kerning_func_t
 </SECTION>
 
 <SECTION>
@@ -271,10 +263,12 @@
 hb_font_funcs_set_glyph_from_name_func
 hb_font_funcs_set_glyph_h_advance_func
 hb_font_funcs_set_glyph_h_advances_func
+hb_font_funcs_set_glyph_h_kerning_func
 hb_font_funcs_set_glyph_h_origin_func
 hb_font_funcs_set_glyph_name_func
 hb_font_funcs_set_glyph_v_advance_func
 hb_font_funcs_set_glyph_v_advances_func
+hb_font_funcs_set_glyph_v_kerning_func
 hb_font_funcs_set_glyph_v_origin_func
 hb_font_funcs_set_nominal_glyph_func
 hb_font_funcs_set_nominal_glyphs_func
@@ -300,8 +294,12 @@
 hb_font_get_glyph_h_advance_func_t
 hb_font_get_glyph_h_advances
 hb_font_get_glyph_h_advances_func_t
+hb_font_get_glyph_h_kerning
+hb_font_get_glyph_h_kerning_func_t
 hb_font_get_glyph_h_origin
 hb_font_get_glyph_h_origin_func_t
+hb_font_get_glyph_kerning_for_direction
+hb_font_get_glyph_kerning_func_t
 hb_font_get_glyph_name
 hb_font_get_glyph_name_func_t
 hb_font_get_glyph_origin_for_direction
@@ -310,6 +308,8 @@
 hb_font_get_glyph_v_advance_func_t
 hb_font_get_glyph_v_advances
 hb_font_get_glyph_v_advances_func_t
+hb_font_get_glyph_v_kerning
+hb_font_get_glyph_v_kerning_func_t
 hb_font_get_glyph_v_origin
 hb_font_get_glyph_v_origin_func_t
 hb_font_get_nominal_glyph