Document return value for git_libgit2_features

include/git2/common.h

@@ -167,6 +167,9 @@ typedef enum {
  * - GIT_FEATURE_SSH
  *   Libgit2 supports the SSH protocol for network operations. This requires
  *   the libssh2 library to be found when compiling libgit2
+ *
+ * - GIT_FEATURE_NSEC
+ *   Libgit2 supports the sub-second resolution in file modification times.
  */
 GIT_EXTERN(int) git_libgit2_features(void);