Rename `block_param` to `block_arg`.

I think `arg` is a more standard name than `param`
for what you pass to a method.
Improve block warning.

- RSpec.warning appends a period, so we should not have one
  on the last sentence or we’ll get a double period.
- The new message is less cryptic and is hopefully more
  clear for end users.
Changelog for #645.
Merge pull request #646 from rspec/645-followups

645 followups