bug#35230: char-displayable-p return code is conflictingly documented

[Eli Zaretskii]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: 35230@debbugs.gnu.org
> Date: Sun, 14 Apr 2019 11:51:36 +0200
> 
>  @defun char-displayable-p char
> -This function returns @code{t} if Emacs ought to be able to display
> -@var{char}.  More precisely, if the selected frame's fontset has a
> -font to display the character set that @var{char} belongs to.
> +This function returns non-@code{nil} if Emacs ought to be able to
> +display @var{char}.  More precisely, if the selected frame's fontset
> +has a font to display the character set that @var{char} belongs to.
>  
>  Fontsets can specify a font on a per-character basis; when the fontset
>  does that, this function's value may not be accurate.
> +
> +This function may return non-@code{nil} even when there is no font
> +available, since it also checks if the coding system for the text
> +terminal can encode the character.
>  @end defun

Thanks.  I think we should tell more about the testing of text
terminal capabilities, because otherwise the reference to terminal
encoding comes out of the blue without being mentioned anywhere else,
since the preceding text talks only about fonts and fontsets,
i.e. only about GUI frames.