Add some missing documentation about return value

include/git2/apply.h

@@ -100,6 +100,7 @@ GIT_EXTERN(int) git_apply_options_init(git_apply_options *opts, unsigned int ver
  * @param preimage the tree to apply the diff to
  * @param diff the diff to apply
  * @param options the options for the apply (or null for defaults)
+ * @return 0 or an error code
  */
 GIT_EXTERN(int) git_apply_to_tree(
 	git_index **out,
@@ -137,6 +138,7 @@ typedef enum {
  * @param diff the diff to apply
  * @param location the location to apply (workdir, index or both)
  * @param options the options for the apply (or null for defaults)
+ * @return 0 or an error code
  */
 GIT_EXTERN(int) git_apply(
 	git_repository *repo,

include/git2/tree.h

@@ -334,6 +334,7 @@ GIT_EXTERN(int) git_treebuilder_insert(
  *
  * @param bld Tree builder
  * @param filename Filename of the entry to remove
+ * @return 0 or an error code
  */
 GIT_EXTERN(int) git_treebuilder_remove(
 	git_treebuilder *bld, const char *filename);
@@ -463,6 +464,7 @@ typedef struct {
  * @param baseline the tree to base these changes on
  * @param nupdates the number of elements in the update list
  * @param updates the list of updates to perform
+ * @return 0 or an error code
  */
 GIT_EXTERN(int) git_tree_create_updated(git_oid *out, git_repository *repo, git_tree *baseline, size_t nupdates, const git_tree_update *updates);