docs: submodule documentation doesn't mention that SubmoduleCollection is a sequence of Submodule instances

I spent a few minutes going over https://www.pygit2.org/submodule.html trying to understand how to get all the submodules from a SubmoduleCollection. It turns out that you can just iterate over it. I think it would be good if the documentation mentioned it (perhaps listing __getitem__ and __iter__ in the list of methods, as is the case in https://www.pygit2.org/objects.html#trees).

Thanks!
Noam

从 pygit2.org/submodule.html 的 Submodule 文档开始，并将其方法列表与 pygit2.org/objects.html#trees 的 Trees 文档进行比较。记录 SubmoduleCollection 是由 Submodule 实例组成的序列，包括其 getitem 和 iter 行为；当用户能够看到如何遍历所有子模块时即完成。