Merge 5.1 into 5.x

[mongodb-php-bot]

Merge new changes from 5.1 into 5.x.

Commits

• Docs changes v5.1 (Docs changes v5.1 #3174): 9108d27

Resolving conflicts

To resolve any conflicts, check out the temporary branch and run the following command:

git merge 5.x

Ignoring changes

To ignore from the remote branch, first reset the temporary branch to 5.x and manually merge using the `ours` merge strategy:

git reset --hard 5.x
git merge --strategy=ours 5.1

Then, push the temporary branch to upate the pull request.