repository: document describe_strategy options

The documentation for Repository.describe() hinted at the GIT_DESCRIBE_*
options constants.

List the valid constants and describe their behavior to make it clearer
for the user.

pygit2/repository.py

@@ -660,7 +660,16 @@ def describe(self, committish=None, max_candidates_tags=None,
  consider. Increasing above 10 will take slightly longer but may
  produce a more accurate result. A value of 0 will cause only exact
  matches to be output.
- :param int describe_strategy: A GIT_DESCRIBE_* constant.
+ :param int describe_strategy: A `GIT_DESCRIBE_*` constant. Can be one
+ of:
+
+ * `GIT_DESCRIBE_DEFAULT` - Only match annotated tags. (This is
+ equivalent to setting this parameter to `None`.)
+ * `GIT_DESCRIBE_TAGS` - Match everything under refs/tags/
+ (includes lightweight tags).
+ * `GIT_DESCRIBE_ALL` - Match everything under refs/ (includes
+ branches).
+
  :param str pattern: Only consider tags matching the given `glob(7)`
  pattern, excluding the "refs/tags/" prefix.
  :param bool only_follow_first_parent: Follow only the first parent