The documentation for time.struct_time (in Doc/library/time.rst) explains tm_isdst as "0, 1 or -1; see below" but then doesn't really go into further detail below, other than to say "A -1 argument as the daylight savings flag, passed to mktime() will usually result in the correct daylight savings state to be filled in.".

In Modules/timemodule.c there's a section which says:
{"tm_isdst", "1 if summer time is in effect, 0 if not, and -1 if unknown"},

IMHO it would be nice if this more accurate info was also present in the HTML documentation.