Flag options in describe.h as being optional

The git_describe_options in git_describe_commit and
git_describe_workdir and the git_describe_format_options in
git_describe_format are optional and can be NULL. State this in the
documentation to make people's lives easier when calling these
functions.

Signed-off-by: Remy Suen <remy.suen@gmail.com>

include/git2/describe.h

@@ -118,7 +118,7 @@ typedef struct git_describe_result git_describe_result;
  * @param result pointer to store the result. You must free this once
  * you're done with it.
  * @param committish a committish to describe
- * @param opts the lookup options
+ * @param opts the lookup options (or NULL for defaults)
  */
 GIT_EXTERN(int) git_describe_commit(
 	git_describe_result **result,
@@ -135,7 +135,7 @@ GIT_EXTERN(int) git_describe_commit(
  * @param out pointer to store the result. You must free this once
  * you're done with it.
  * @param repo the repository in which to perform the describe
- * @param opts the lookup options
+ * @param opts the lookup options (or NULL for defaults)
  */
 GIT_EXTERN(int) git_describe_workdir(
 	git_describe_result **out,
@@ -148,7 +148,7 @@ GIT_EXTERN(int) git_describe_workdir(
  * @param out The buffer to store the result
  * @param result the result from `git_describe_commit()` or
  * `git_describe_workdir()`.
- * @param opts the formatting options
+ * @param opts the formatting options (or NULL for defaults)
  */
 GIT_EXTERN(int) git_describe_format(
 	git_buf *out,