Update docscheck.sh to accept provider function docs

[SarahFrench]

(This docscheck.sh script is a good candidate for being replaced with official tools, but for now updating this script to avoid false alarms about bad docs)

After merging this PR I'll cherry pick the commit from main into the feature branch.

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Manual test without this edit to docscheck.sh:

• If I add the file website/docs/functions/example.html.markdown with example docs content from here to the TPG repo
• and then run make docscheck
• I see:

terraform-provider-google % make docscheck
Unknown category "functions".  Docs can only exist in r/, d/, or guides/ folders.
make: *** [docscheck] Error 1

Manual test with this PR's edit to docscheck.sh:

• Given a repo with the new file described above
• If I update the script as per this PR and then run make docscheck I see no output:

terraform-provider-google % make docscheck

[github-actions]

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[SarahFrench]

I'm going to proceed without review/approval as I've tested the change above and the strategy for these PRs was already discussed here: #17465 (comment)

[github-actions]

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