cmd/api: api check's false positive when "Deprecated:" text comes up in the middle of a paragraph instead of at the beginning

[dmitshur]

api check implements its own logic to detect whether an exported API symboc is marked as deprecated. It can report a false positive when a "Deprecated:" string comes up in the middle of a paragraph, not at the beginning of a paragraph. Also see #79139 (comment).

[gopherbot]

Change https://go.dev/cl/774880 mentions this issue: cmd/api/testdata: add test case for issue 79145

[gopherbot]

Change https://go.dev/cl/774881 mentions this issue: cmd/api: fix false positive in isDeprecated