doc: document NetHSM client update procedure

[nponsard]

This PR documents the steps to take to update the NetHSM client

Changes

• document NetHSM client update procedure
• Change the makefile so make nethsm-api.yml always downloads the newer version

(I don't have PGP signing configured, sorry for that)

[nponsard]

I removed the Makefile modification from the commit and updated the instructions