(eldoc-get-fnsym-args-string): Convert

(eldoc-get-fnsym-args-string): Convert
comment to basic doc string.  Also apply eldoc-argument-case in
the help-split-fundoc case.  Adapt for changed behavior of
eldoc-function-argstring, eldoc-function-argstring-format, and
eldoc-highlight-function-argument.
(eldoc-highlight-function-argument): Make INDEX argument
optional, just call eldoc-docstring-format-sym-doc if absent.
(eldoc-function-argstring): Change the behavior.  Now it converts
an argument list to a string.
(eldoc-function-argstring-format): Change the behavior.  Now it
applies `eldoc-argument-case' to a string.