[9.20] fix: doc: nsupdate does not handle zero length RDATA well

[9.20] fix: doc: nsupdate does not handle zero length RDATA well

Nsupdate does not distinguish between a non-existing RDATA field
and an empty RDATA field when determining which action is desired
when the RDATA field is empty. This only affects a few data types,
like APL, which allow an empty RDATA field.  Document a workaround
of using the '\# 0' form for entering these specific records. e.g.

    # delete the APL RRset
    update delete IN APL
    # delete the APL record with a zero length rdata
    update delete IN APL \# 0

Closes #5835

Backport of MR !11775

Merge branch 'backport-5835-nsupdate-doc-zero-length-rdata-how-to-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11823