HTML API: Clarify parse_next_attribute() return value

[sirreal]

Docs-only follow-up to r62595 (#12319), which rolled back a change to the parse_next_attribute() @return docs so it wouldn't hold up the fix. See the review discussion: #12319 (comment)

The existing line, "Whether an attribute was found before the end of the document," names only one reason for a false return. The common one is reaching the end of the tag. This keeps the positive "whether an attribute was found" framing and names both stop conditions.

Trac ticket: https://core.trac.wordpress.org/ticket/65860

Follow-up to r62595 (see #65372).

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Fable 5
Used for: Drafting the @return text from the linked review discussion; reviewed and edited by me.

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