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.

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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, 1 insertions, 1 deletions

diff --git a/railties/CHANGELOG.md b/railties/CHANGELOG.md
index da11f337ad..6193c33593 100644
--- a/railties/CHANGELOG.md
+++ b/railties/CHANGELOG.md
@@ -2,7 +2,7 @@
 
     *Islam Wazery*
 
-*   Print `bundle install` output in `rails new` as soon as it's available
+*   Print `bundle install` output in `rails new` as soon as it's available.
 
     Running `rails new` will now print the output of `bundle install` as
     it is available, instead of waiting until all gems finish installing.