docs(webc): Formatting tweaks

[Zearin]

Mainly wanted to add a little clarity to some code examples that did not specify whether the input was from a page or a component.

I found myself reading and re-reading these docs because I wasn’t sure if certain webc:* attributes only applied in component files or page files.

Some examples also included the input code and output code in the same block. That was understandable, but it’s also inconsistent with the way most other examples are shown throughout the docs. I tried to bring these cases more in line with typical usage.

I also noticed one paragraph that should probably be a callout, so I added the codes to make it so.

Finally, there are a few commas and other small copy-edits to the text itself.

[Zearin]

@zachleat ping

[Zearin]

@zachleat ping (6 mo; easy merge 🤓)