Clarify Redis cache store docs

[swrobel]

This should be backported to 5-2-stable along with the other change made to these docs.

Summary

1. Fix minor grammatical issues & expound on some potentially confusing sections
2. Explain hiredis support
3. Add basic configuration
4. Update example production config to fix syntax errors:

# frozen_string_literal: true

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  # Activate the gem you are reporting the issue against.
  gem "rails", "5.2.0.rc1"
end

require "minitest/autorun"

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

Rails.application.configure do |config|
  # http://edgeguides.rubyonrails.org/caching_with_rails.html#activesupport-cache-rediscachestore
  cache_servers = %w[ "redis://cache-01:6379/0", "redis://cache-02:6379/0", … ],
  config.cache_store = :redis_cache_store, url: cache_servers,

  connect_timeout:    30,  # Defaults to 20 seconds
  read_timeout:       0.2, # Defaults to 1 second
  write_timeout:      0.2, # Defaults to 1 second
  reconnect_attempts: 1,   # Defaults to 0

  error_handler: -> (method:, returning:, exception:) {
    # Report errors to Sentry as warnings
    Raven.capture_exception exception, level: 'warning",
      tags: { method: method, returning: returning }
  }
end

$ ruby -v
ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin17]
$ ruby test.rb  
test.rb:26: syntax error, unexpected tLABEL
...tore = :redis_cache_store, url: cache_servers,
...                           ^~~~

[jeremy]

Great edits. Thanks @swrobel!