Align incoming email token capability across UI and docs

What does this MR do and why?

Aligns the documentation and UI copy that describes the incoming email token so that every surface consistently and accurately reflects how the token works.

Previously, different places described the token's behavior inconsistently. This updates the copy so it consistently states what that the incoming email token can be used for across the documentation and UI, giving users an accurate understanding of what the token does and where its addresses appear.

In additional aligning the copy, this also makes it clearer that the incoming email token is a token that should be kept private and protected, and documents how to reset it more clearly.

Surfaces updated:

• Personal Access Tokens settings page (incoming email token description)
• The "Email work item to this project" modal
• Token overview, issue creation, group access, and secret-detection documentation

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