Brush up doc strings and terminology in plstore.el

Brush up doc strings and terminology in plstore.el

* lisp/plstore.el (plstore-encoded, plstore-passphrase-callback-function)
(plstore--init-from-buffer, plstore-revert, plstore-close)
(plstore--merge-secret, plstore--decrypt, plstore--match, plstore-find)
(plstore-get, plstore-put, plstore-delete, plstore--insert-buffer)
(plstore-save, plstore--encode, plstore--decode)
(plstore--write-contents-functions, plstore-mode-decoded)
(plstore-mode): Brush up doc strings and documentation in general.
Fix terminology, in particular spurious occurences of all uppercase
"PLSTORE".  (Bug#63627)