Notes regarding the removal of specific items:
- Aztec C: only on the Amiga.
- mch_check_win(): doesn't exist anymore.
- Comment in ex_cmds.c: It seems the context for this comment was
removed, but the comment was inadvertantly left alone.
Closes https://github.com/neovim/docs/pull/26
Also added stubs for 'cryptmethod' and 'key', and placeholders for
explanation regarding removal of crypto functionality.