Improve documentation of CXIndexOptions; NFC

Document one more alternative way to initialize struct CXIndexOptions,
which is used in LibclangSetPreambleStoragePathTest since
df8f8f7 because the previous
initialization approach causes compiler warnings.

Differential Revision: https://reviews.llvm.org/D145775

clang/include/clang-c/Index.h

@@ -329,8 +329,8 @@ typedef enum {
  * Index initialization options.
  *
  * 0 is the default value of each member of this struct except for Size.
- * Initialize the struct in one of the following two ways to avoid adapting code
- * each time a new member is added to it:
+ * Initialize the struct in one of the following three ways to avoid adapting
+ * code each time a new member is added to it:
  * \code
  * CXIndexOptions Opts;
  * memset(&Opts, 0, sizeof(Opts));
@@ -340,6 +340,11 @@ typedef enum {
  * \code
  * CXIndexOptions Opts = { sizeof(CXIndexOptions) };
  * \endcode
+ * or to prevent the -Wmissing-field-initializers warning for the above version:
+ * \code
+ * CXIndexOptions Opts{};
+ * Opts.Size = sizeof(CXIndexOptions);
+ * \endcode
  */
 typedef struct CXIndexOptions {
   /**