Re: restore_command return code behaviour

I agree that reusing archive_command's wording is probably the way to
go. I think archive_cleanup_command and recovery_end_command have the
same issue; opinions on changing those as well?

Nitpick mode: I think the current wording for archive_command could be
misleading.

> an error by the shell with an exit status greater than 125 (such as command not found)

The phrase "by the shell" is not really true here (that's kind of the
point of the thread, I'd argue) and I'm wondering if we should move
that to the parenthetical. But I don't like any of my draft ideas so
far, and I don't want to get in the way of a small improvement by
demanding perfection.

The recovery will be aborted and the server will stop if any of the following events occur:
- the command was terminated by a signal other than SIGTERM (which is used as part of a database server shutdown);
- the command returns an exit code greater than 125
- the shell returns an error (such as 'command not found')

The former has a 'heavier' style; the latter has the benefit of clearly showing each condition for shutting down the server (but it breaks the GUC style, where bullet points are only used for defining possible values).