Clarify :submodules use

[malhal]

Hi orta, hope you had a nice time in SF! After doing a few hours research into getting my pod lib to work with a built-in git submodule it appears to me the specs page could be improved better to explain the :submodules param. It could be explained before talking about "subspecs" since it has nothing to do with that and actually adds to the confusion on what it is for. And also it should be noted that if using a pod :git line in a Podfile the :submodules in the spec is ignored. Then in subspecs section it should be explained why the Facebook subspec needs :submodules set, I'm not sure why yet though maybe something to do with its dependency. Happy to PR.