Improve doc strings in textsec.el

Improve doc strings in textsec.el

* lisp/international/textsec.el (textsec-mixed-numbers-p)
(textsec-ascii-confusable-p, textsec-resolved-script-set)
(textsec-single-script-confusable-p)
(textsec-mixed-script-confusable-p)
(textsec-whole-script-confusable-p, textsec-domain-suspicious-p)
(textsec-local-address-suspicious-p, textsec-name-suspicious-p)
(textsec-suspicious-nonspacing-p): Doc fix.