Improved documentation for RSpec/ChangeByZero

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

Since it isn’t mentioned that we define our own negation matcher
for `not_change` for the compound expectations case,

I felt it would be a bit confusing,
so I'll explicitly mention that define a negation matcher
for the good case.

```ruby
define_negated_matcher :not_change, :change
expect { run }
  .to not_change(Foo, :bar)
  .and not_change(Foo, :baz)
expect { run }
  .to not_change { Foo.bar }
  .and not_change { Foo.baz }
```

Author: ydah

docs/modules/ROOT/pages/cops_rspec.adoc

@@ -404,6 +404,8 @@ Prefer negated matchers over `to change.by(0)`.
 # bad
 expect { run }.to change(Foo, :bar).by(0)
 expect { run }.to change { Foo.bar }.by(0)
+
+# bad - compound expectations
 expect { run }
   .to change(Foo, :bar).by(0)
   .and change(Foo, :baz).by(0)
@@ -414,6 +416,9 @@ expect { run }
 # good
 expect { run }.not_to change(Foo, :bar)
 expect { run }.not_to change { Foo.bar }
+
+# good - compound expectations
+define_negated_matcher :not_change, :change
 expect { run }
   .to not_change(Foo, :bar)
   .and not_change(Foo, :baz)

lib/rubocop/cop/rspec/change_by_zero.rb

@@ -9,6 +9,8 @@ module RSpec
       #   # bad
       #   expect { run }.to change(Foo, :bar).by(0)
       #   expect { run }.to change { Foo.bar }.by(0)
+      #
+      #   # bad - compound expectations
       #   expect { run }
       #     .to change(Foo, :bar).by(0)
       #     .and change(Foo, :baz).by(0)
@@ -19,6 +21,9 @@ module RSpec
       #   # good
       #   expect { run }.not_to change(Foo, :bar)
       #   expect { run }.not_to change { Foo.bar }
+      #
+      #   # good - compound expectations
+      #   define_negated_matcher :not_change, :change
       #   expect { run }
       #     .to not_change(Foo, :bar)
       #     .and not_change(Foo, :baz)