Improve documentation of CXIndexOptions; NFC

[vedgy]

Summary

Document one more alternative way to initialize struct CXIndexOptions,
which is used in LibclangSetPreambleStoragePathTest since
df8f8f76207df40dca11c9c0c2328d6b3dfba9ca because the previous
initialization approach causes compiler warnings.

clang/include/clang-c/Index.h

CXGlobalOpt_ThreadBackgroundPriorityForAll =
       CXGlobalOpt_ThreadBackgroundPriorityForEditing
 
 } CXGlobalOptFlags;
 
 /**
  * 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));
  * Opts.Size = sizeof(CXIndexOptions);
  * \endcode
  * or explicitly initialize the first data member and zero-initialize the rest:
  * \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 {
   /**
    * The size of struct CXIndexOptions used for option versioning.
    *
    * Always initialize this member to sizeof(CXIndexOptions), or assign
    * sizeof(CXIndexOptions) to it right after creating a CXIndexOptions object.
    */