(eldoc-last-data): Revise documentation.

(eldoc-last-data): Revise documentation.
(eldoc-print-current-symbol-info): Adjust for changed helper
function signatures.
(eldoc-get-fnsym-args-string): Add `args' argument.  Use new
`eldoc-highlight-function-argument'.
(eldoc-highlight-function-argument): New function.
(eldoc-get-var-docstring): Format documentation with
`font-lock-variable-name-face'.
(eldoc-docstring-format-sym-doc): Add `face' argument and apply it
where suited.
(eldoc-fnsym-in-current-sexp): Return a list with argument index.
(eldoc-beginning-of-sexp): Return number of skipped sexps.