Fix typos and improve the documentation

This is a squash of the following commits, from first to last:

-

Fix minor, random things I’ve come across lately that individually
did not seem worth making a PR for, so I saved them for one commit.

One common error is using “it’s” (which is an abbreviation of “it is”)
when the possessive “its” should be used for indicating possession.

-

Changes include the name of a test, so remove the `[skip ci]` (thanks @senny).

-

Line wrap the changes at 80 chars and add one more doc fix.

-

Add a missing line wrap in the Contributing to Ruby on Rails Guide.

-

Line wrap the `TIP` section in the Contributing to Ruby on Rails Guide as well.

Rendering the guide locally with `bundle exec rake guides:generate` did
not show any change in on-screen formatting after adding the line wrap.

The HTML generated is (extra line added to illustrate where the line
wrap takes place):

    <div class="info"><p>Please squash your commits into a single commit
when appropriate. This

    simplifies future cherry picks and also keeps the git log
clean.</p></div>

-

Squash commits.

1 files changed, 3 insertions, 3 deletions

diff --git a/activesupport/lib/active_support/core_ext/module/concerning.rb b/activesupport/lib/active_support/core_ext/module/concerning.rb
index 07a392404e..e26b594fc4 100644
--- a/activesupport/lib/active_support/core_ext/module/concerning.rb
+++ b/activesupport/lib/active_support/core_ext/module/concerning.rb
@@ -63,10 +63,10 @@ class Module
   #
   # == Mix-in noise exiled to its own file:
   #
-  # Once our chunk of behavior starts pushing the scroll-to-understand it's
+  # Once our chunk of behavior starts pushing the scroll-to-understand-it
   # boundary, we give in and move it to a separate file. At this size, the
-  # overhead feels in good proportion to the size of our extraction, despite
-  # diluting our at-a-glance sense of how things really work.
+  # increased overhead can be a reasonable tradeoff even if it reduces our
+  # at-a-glance perception of how things work.
   #
   #   class Todo
   #     # Other todo implementation