make rustdoc::invalid_html_tags more robust

[lolbinarycat]

best reviewed a commit at a time.

first commit eliminates a false negative, second commit eliminates a false positive.

for some reason, pulldown-cmark splits html blocks into their constituent lines, so we just coalesce the iterator to recombine these.. this approach might eliminate other bugs if we did it elsewhere, but i'm not sure what or where.

fixes #145529

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[klensy]

pulldown-cmark splits html blocks

Used pulldown-cmark is slightly old for reasons, this can be changed in more recent versions.