Fix: Clarify signature property controls sender display name for all affected versions (Product IS issue #26144)

[wso2-engineering-bot]

This PR was automatically generated by Claude AI.

• Issue: Clarify output_adapter.email.signature property controls the sender display name product-is#26144
• Type: Documentation / Suggestions
• Summary: Updated the ambiguous description of the output_adapter.email.signature property to clarify that it controls the sender's display name, not an email signature block. The property is now documented as specifying the display name for the "From" email address, with clear notes about SMTP server behavior when the property is unset or set to an empty string.
• Affected Versions: 5.10.0, 5.11.0, 6.0.0, 6.1.0, 7.0.0, 7.1.0, 7.2.0, next
• Style Scope Verification: Microsoft Style Guidelines have been applied ONLY to newly added content without modifying existing content style unless specifically requested.
• Verification: mkdocs build passed successfully

Changes Made:

• Updated the description of signature property in [output_adapter.email] configuration across all affected versions
• Clarified that the property specifies the display name for the "From" email address
• Added notes about SMTP server behavior when the property is left unset (defaults to null, may be replaced by authentication username)
• Documented how to send emails with no display name (set to empty string)
• Updated both global configuration and per-tenant configuration sections where applicable

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