During an internal walkthrough of the Web embedding documents, we came up with the following action items:

• Document the need for the runWidget function
• Rename "Full-screen" to "Full page"
• Tweak the title to be smarter, "embedding" is not very meaningful

One of our users was also running the docs, and encountered some of the issues that this PR addresses:

• Fixes: Unexpected null value error appears when running multiView on Flutter Web flutter#153198

There's also some minor typo/formatting fixes.

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.