Improved documentation for RSpec/ExpectActual

This PR is improves documentation for `RSpec/ExpectActual`.

For example, if you create a gem by `bundle gem hoge`, the following rspec code will be created.

```ruby
it "does something useful" do
  expect(false).to eq(true)
end
```

At first glance, it seems to be autocorrected,
but when both expected and actual values are literal,
no autocorrection is performed.
We'll add this as an explanation for cop.

Author: ydah

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -1483,6 +1483,8 @@ end
 
 Checks for `expect(...)` calls containing literal values.
 
+Autocorrection is performed when the expected is not a literal.
+
 === Examples
 
 [source,ruby]

lib/rubocop/cop/rspec/expect_actual.rb

@@ -5,6 +5,8 @@ module Cop
     module RSpec
       # Checks for `expect(...)` calls containing literal values.
       #
+      # Autocorrection is performed when the expected is not a literal.
+      #
       # @example
       #   # bad
       #   expect(5).to eq(price)