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: #25672
Fixes: #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>

doc/api/repl.md

@@ -540,8 +540,10 @@ environment variables:
 
  - `NODE_REPL_HISTORY` - When a valid path is given, persistent REPL history
    will be saved to the specified file rather than `.node_repl_history` in the
-   user's home directory. Setting this value to `''` will disable persistent
-   REPL history. Whitespace will be trimmed from the value.
+   user's home directory. Setting this value to `''` (an empty string) will
+   disable persistent REPL history. Whitespace will be trimmed from the value.
+   On Windows platforms environment variables with empty values are invalid so
+   set this variable to one or more spaces to disable persistent REPL history.
  - `NODE_REPL_HISTORY_SIZE` - Controls how many lines of history will be
    persisted if history is available. Must be a positive number.
    **Default:** `1000`.