With this commit we update the docs according to the latest changes that
were necessary to support loop and pool (which requires all 255
internally used accounts to be imported at wallet creation time).
Fixes#7567.
This commit adds a short guide that explains how a remote signing node
setup should be migrated from lnd v0.14.x-beta to lnd v0.15.x-beta and
adds a note to all 0.15.x release notes.