Wrap before & after words in quotes when they're referring to filters

Using these words without quotes (example sentence: Before filters are run) can be confusing to a newcomer.
Add a note for the example in Setting Custom Headers
Slightly improve the syntax & grammar
Remove unecessary word
Some minor fixes
Make XmlMini.with_backend usable with threads

`XmlMini.with_backend` now may be safely used with threads:

  Thread.new do
    XmlMini.with_backend("REXML") { rexml_power }
  end
  Thread.new do
    XmlMini.with_backend("LibXML") { libxml_power }
  end

Each thread will use it's own backend.
Add a consideration note

I believe this paragraph should move to the end as it's a little out of the scope of Rails guides.
Some minor fixes & improvements
Move "Using Action View outside of Rails" at the bottom

See 8db41ff7698eed542fb09821de37fc7ea1f2bd5d why
Improve the starting sentence
Update table of contents as a result of #bf63c4971d73e80739c9f2619a9fa0dfb2223435
Merge pull request #8219 from nikitug/threadsafe_xmlmini_with_backend

Make XmlMini.with_backend usable with threads

Conflicts:
	activesupport/CHANGELOG.md
Minor improvements & fixes
Minor improvements & fixes
Fix AS::Concern example (current example doesn't work)
Fix another AS::Concern example (class_eval is already called by included)
Update links to API documentation & add a TODO
Minor improvements & fixes
Some minor improvements
Add links to TryGit & official documentation