Correct the docblock description for is_single()

[johnbillion]

The docblock description for is_single() states:

Is the query for an existing single post?

Works for any post type, except attachments and pages

It appears that the "Works for" phrase is incorrect and has been for the last six years (since [16830]) because is_single() returns true for attachment views. Either that, or the function's behaviour changed at some point before its tests were introduced.

Tests for is_single() on an attachment view have been in place for at least three years (since [25002]).

Originally reported by @danburzo in Slack.

[ocean90]

Duplicate of #38225.