[docs] Reduce warnings

src/hb-paint.h

@@ -146,7 +146,7 @@ typedef void (*hb_paint_pop_transform_func_t) (hb_paint_funcs_t *funcs,
  *
  * A virtual method for the #hb_paint_funcs_t to render a color glyph by glyph index.
  *
- * Return value: %true if the glyph was painted, %false otherwise.
+ * Return value: `true` if the glyph was painted, `false` otherwise.
  *
  * Since: 8.2.0
  */

src/hb-sanitize.hh

@@ -72,8 +72,8 @@
  *
  * === The sanitize() contract ===
  *
- * The sanitize() method of each object type shall return true if it's safe to
+ * The sanitize() method of each object type shall return `true` if it's safe to
- * call other methods of the object, and %false otherwise.
+ * call other methods of the object, and `false` otherwise.
  *
  * Note that what sanitize() checks for might align with what the specification
  * describes as valid table data, but does not have to be.  In particular, we