Clarify behavior of json_escape, update examples

The behavior of json_escape was fixed in 2f1c5789, but the doc
changes and example in that commit incorrectly indicated that the
return value would be html-safe. Since quotation marks are
preserved, the raw value is not safe to use in other contexts
(specifically HTML attributes).
Skip Spring App Generator tests on JRuby

Spring makes extensive use of Process.fork, so won't be able to provide
JRuby Support, hence skip these tests on JRuby.
Merge pull request #13646 from jenseng/json_escape

Clarify behavior of json_escape [ci skip]
Merge pull request #13687 from gaurish/spring-has-no-love-for-jruby

Skip Spring App Generator tests on JRuby
doc proc/lambda arg on inclusion validation. Closes #13689. [ci skip]
highlight http://localhost:3000 in README.md. Closes #13643. [ci skip]

This has no effect on the API docs. The main API page is taken from:
    railties/RDOC_MAIN.rdoc
fix bug in becomes! when changing from base to subclass. Closes #13272.
standardize on jruby_skip & rbx_skip

This Adds helpers(jruby_skip & rbx_skip). In Future, Plan is to use
these helpers instead of calls directly to
RUBY_ENGINE/RbConfig/JRUBY_VERSION
Merge pull request #13693 from gaurish/typo

[JRuby] Fix incorrect RUBY_ENGINE value
Set NameError#name