doc: fix comment on GIT_EUSER

Author: tiennou

include/git2/errors.h

@@ -27,7 +27,8 @@ typedef enum {
 	GIT_EAMBIGUOUS = -5,		/**< More than one object matches */
 	GIT_EBUFS      = -6,		/**< Output buffer too short to hold data */
 
-	/* GIT_EUSER is a special error that is never generated by libgit2
+	/**
+	 * GIT_EUSER is a special error that is never generated by libgit2
 	 * code.  You can return it from a callback (e.g to stop an iteration)
 	 * to know that it was generated by the callback and not by libgit2.
 	 */