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.

0 files changed, 0 insertions, 0 deletions