Undocumented behaviour of \s in SIMILAR TO expression

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/11/functions-matching.html
Description:

SELECT ' ' SIMILAR TO '[\s]';
returns true

SELECT ' ' SIMILAR TO '[\q]';
generates an error: [2201B]: ERROR: invalid regular expression: invalid
escape \ sequence

\s is not mentioned in the SIMILAR TO section
(https://www.postgresql.org/docs/current/functions-matching.html#FUNCTIONS-SIMILARTO-REGEXP).
If it’s officially supported (which would be useful for my current task), it
should be documented. Otherwise the server should generate an error like for
other invalid escapes.