fix typo
doc: clean up docstring for SettingsManager
doc: minor tweak of interface doc
doc: fix Envelope.attachments docstring
doc: clean up crypto docs
doc: move crypto docs to usage section
doc: update faq
doc: fix missing section names

in usage/commands/*
doc: clean up error message for invalid subcommands
Merge branch '0.3.4-fix-docs' into testing