address '--disable-doh' failures

Change 5756 (GL #2854) introduced build errors when using
'configure --disable-doh'.  To fix this, isc_nm_is_http_handle() is
now defined in all builds, not just builds that have DoH enabled.

Missing code comments were added both for that function and for
isc_nm_is_tlsdns_handle().
68 jobs for !5561 with each-fix-warnings in 22 minutes and 11 seconds (queued for 11 seconds)
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