Introduces -> Introduce
returns -> return

We say it's useful for performing version comparison. Let's show an example. A good one is how `Rails.version > '4.1.10'` would be true for `'4.1.2'` also due to lexicographic string comparison, but would be false when comparing gem versions.

Unfortunately you can't do `Rails.gem_version > '4.1.10'`. You have to:
```ruby
if Rails.gem_version > Gem::Version.new('4.1.10')
  # ...
```
and that's limited to exact version comparison. Pretty verbose and hard to discover.

Furthermore, to do more realistic version requirement checks, you'd use:
```ruby
if Gem::Requirement.create('~> 4.1.2') =~ Rails.gem_version
  ...
```
Which you'd need some digging in RubyGems docs to even figure out.

All told, we may be better off just saying "Introduce `Rails.gem_version` as a convenience method to return `Gem::Version.new(Rails.version)`. It's not something most app developers would ever touch, so suggesting it's useful is a stretch, but plugin developers may take note and use it.