[ruby-mode] Better slurping and barfing + missing docstrings.

Added a better `delete-indentation` that is ruby-aware. Doing that, slurping and barfing work much better.

Before:
```ruby
begin
end
test(1).test[2].test

# 3x forward slurp

begin
  test (1) .test [2] .test
end
```

After:
```ruby
begin
end
test(1).test[2].test

# 3x forward slurp

begin
  test(1).test[2].test
end
```