bug#31376: 26.0.50; print-charset-text-property not honored

[Drew Adams]

> We should also mention in the doc
> string that any non-nil, non-t value is treated as 'default'.

That doesn't sound great - not very conventional.
Instead of `t' meaning something special and other
non-nil meaning something non-special we usually
have a mnemonic symbol for the special case and
other non-nil for the non-special case. 

It's no doubt too late to change this, since the
mistake seems to have been made way back in Emacs 23.

The doc should even point out that this is maybe not
what you might expect.  IOW, stress that `t' is a
special case.