Restore docs for \char_<thing>case:N

l3kernel/l3token.dtx

@@ -151,6 +151,26 @@
 %   \end{texnote}
 % \end{function}
 %
+% \begin{function}[added = 2020-01-09, EXP]
+%   {
+%     \char_lowercase:N, \char_uppercase:N,
+%     \char_titlecase:N, \char_foldcase:N,
+%     \char_str_lowercase:N, \char_str_uppercase:N,
+%     \char_str_titlecase:N, \char_str_foldcase:N
+%   }
+%   \begin{syntax}
+%     \cs{char_lowercase:N} \meta{char}
+%   \end{syntax}
+%   Converts the \meta{char} to the equivalent case-changed character
+%   as detailed by the function name (see \cs{str_foldcase:n}
+%   and \cs{text_titlecase:n} for details of these terms). The case mapping
+%   is carried out with no context-dependence (\emph{cf.}~\cs{text_uppercase:n},
+%   \emph{etc.}) The \texttt{str} versions always generate \enquote{other}
+%   (category code $12$) characters, whilst the standard versions generate
+%   characters with the category code of the \meta{char} (i.e.~only
+%   the character code changes).
+% \end{function}
+%
 % \begin{variable}[added = 2011-09-05]{\c_catcode_other_space_tl}
 %   Token list containing one character with category code $12$,
 %   (\enquote{other}), and character code $32$ (space).