docs: Clarify support for multiple TXT record strings
docs: Clarify support for multiple TXT record strings

Most TXT records only contain a single string, but some may contain
more.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1760

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib