Fixes Wilfred#327

Looks like I should have pulled before tagging a release

I accidentally released 0.20 on an older checkout, so 0.21 has been
released today too.

Uses read-char-choice for selecting between callable (press "c") and
variable (press "v") when the symbol is ambiguous, eg.
`buffer-file-name'.

Less mental effort required than parsing a y-or-n-p prompt and answering
in the negative to "show variable?" to show help for a function.

Uses the native `describe-face`, possible to switch to a `helpful-face`
command if implemented in the future.

User options are variables declared with defcustom. It is useful to
know if a variable is a user option or not, because if it isn't, it's
usually not safe to set it in the init file.

Calling user options "customizable variables" makes it clear that a
user option is still a variable. This is kind of like how interactive
functions are not referred to as "commands" in the summary.

…vely

Fixes Wilfred#326