[backport#14934] Descriptor expansion cache clarifications

Summary:
2e68ffaf205866e4cea71f64e79bbfb89e17280a [doc] descriptor: explain GetPubKey() usage with cached public key (Sjors Provoost)
2290269759ad10cc2e35958c7b0a63f3a7608621 scripted-diff: rename DescriptorImpl m_script_arg to m_subdescriptor_arg (Sjors Provoost)

Pull request description:

I found the name `m_script_arg` to be confusing while reviewing https://github.com/bitcoin/bitcoin/pull/14646#discussion_r240677238. @sipa let me know if `m_subdescriptor_arg` is completely wrong.

I also added an explanation of why we call `GetPubKey` when we don't ask it for a public key.

ACKs for top commit:

laanwj:
  ACK 2e68ffaf205866e4cea71f64e79bbfb89e17280a

Backport of Core PR14934

Test Plan:

ninja all check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7806