Add WordPress guidance for iframes lazy-loading

[JayHoltslander]

Add paragraph with bulk update solution for Wordpress users

next step: @kaycebasques to verify that we support this approach (pinged relevant people on 8/25)

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[JayHoltslander]

@googlebot I signed it!

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[robdodson]

@addyosmani are you ok reviewing this change?

[robdodson]

See #3705 for an explanation on why this is failing. I think rebasing or pulling from master will fix it.

[kaycebasques]

This is a wonderful little addition, thanks @JayHoltslander (you seem to have written technical docs before)

I'm going to get someone in to verify that we recommend this WP practice

[JayHoltslander]

Accept suggestions

[kaycebasques]

Someone pointed out to me that we're not formatting WordPress correctly

(Tip: Use the Add suggestion to batch feature to batch-accept all of these small edits as one commit)

[felixarntz]

Mostly LGTM. I added a reference to the WP core ticket where this is being worked on and updated the code snippet to cover for once that is built into core, to avoid this snippet from conflicting.

[addyosmani]

Similarly, change mostly LGTM.