OpenSSH 9.0 released

The way -a is documented is an example of how it's really difficult to write documentation that's both clear to newcomers and concise for experts.

When you look further down (I have 3.2.3 from Aug 2020), it says:

       --archive, -a
              This is equivalent to -rlptgoD.  It is a quick way of saying you want recursion and want to preserve almost every‐
              thing (with -H being a notable omission).  The only exception to the above equivalence  is  when  --files-from  is
              specified, in which case -r is not implied.

              Note that -a does not preserve hardlinks, because finding multiply-linked files is expensive.  You must separately
              specify -H.

This is much clearer if you're not used to rsync, but a lot less concise - it tries to explain the effect of -rlptgoD but not -H -A -X. If you're an expert user just re-checking the options, though, you'd see the -rlptgoD bit and mentally expand it to: --archive does all the things you'd expect if you're using rsync as a rcp replacement - it handles directories recursively, it copies symlinks as symlinks, it preserves permissions, mtime, group and owner, and it correctly handles Devices and special files, but it does not attempt to copy Hard links as hard links, ACLs or Xattrs.