Update some language in WebAssembly concepts

[pdubroy]

Description

• Change first line of "What is WebAssembly" to be more precise and avoid duplicating language from the main WebAssembly page.
• Update a couple sentences to reflect the current state of Wasm

Motivation

• Clarify current state and add more detail

Additional details

N/A

Related issues and pull requests

[github-actions]

Preview URLs (1 page)

• /en-US/docs/WebAssembly/Guides/Concepts

External URLs (2)

URL: /en-US/docs/WebAssembly/Guides/Concepts
Title: WebAssembly concepts

• https://webassembly.org/docs/non-web/ (1 time) (Note! This may be a new URL 👀)
• https://www.w3.org/community/webassembly/ (1 time) (Note! This may be a new URL 👀)

[chrisdavidmills]

Thanks, @pdubroy, this is really helpful.

I am currently working on a large update to the MDN Wasm docs, particularly around documenting all the missing text instructions, so any help with improving the accuracy of the existing material is much appreciated.