repl: improve doc for disabling REPL history on Windows

Environment variables with empty values are not permitted on Windows. As
such, to disable persistent REPL history one or more spaces should be
used. Node will trim whitespace from the variable, resulting in a blank
variable at runtime and the desired behaviour.

PR-URL: https://github.com/nodejs/node/pull/25672
Fixes: https://github.com/nodejs/node/issues/25661


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>