doc: add documentation to all the public structs and enumscmn/doc-all

This makes them show up in the reference, even if the text itself isn't
the most descriptive.

These have been found with

    grep -Przon '\n\ntypedef struct.*?\{' -- include
    grep -Przon '\n\ntypedef enum.*?\{' -- include

1 files changed, 6 insertions, 1 deletions

diff --git a/include/git2/common.h b/include/git2/common.h
index 1363316b6..d84a76512 100644
--- a/include/git2/common.h
+++ b/include/git2/common.h
@@ -123,7 +123,12 @@ typedef enum {
  */
 GIT_EXTERN(int) git_libgit2_features(void);
 
-
+/**
+ * Global library options
+ *
+ * These are used to select which global option to set or get and are
+ * used in `git_libgit2_opts()`.
+ */
 typedef enum {
 	GIT_OPT_GET_MWINDOW_SIZE,
 	GIT_OPT_SET_MWINDOW_SIZE,