Add at-mention to word list

What does this MR do?

I propose to add @mention to the A-Z word list.

In our docs, we've have a mix of formatting it with code formatting and as regular word.

Google style guide advises to avoid using code-formatted words like real words (ref):

Not recommended: Retrieve information by GETting the data.

As we use @mention as a verb, and so, will sometimes conjugate it, let's drop the code formatting.

Neither Google or Microsoft style guides describe @mention usage specifically.

Links:

• Here MSFT Support uses it like a regular word: https://support.microsoft.com/en-us/office/use-mentions-to-get-someone-s-attention-90701709-5dc1-41c7-aa48-b01d4a46e8c7.
• This Wikipedia page also doesn't use any special formatting: https://en.wikipedia.org/wiki/Mention_(blogging)#@_(at_sign)

Related issues

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