C API: document sk_matrix_t

Change-Id: I8f35868de6ef7d541cc3c9bcaddad4871bf8d6eb
Reviewed-on: https://skia-review.googlesource.com/2590
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Reed <reed@google.com>
1 file changed