doc: add note about mkdtemp() platform differences

PR-URL: #26944
Fixes: #26435
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>

doc/api/fs.md

@@ -2277,7 +2277,10 @@ changes:
 Creates a unique temporary directory.
 
 Generates six random characters to be appended behind a required
-`prefix` to create a unique temporary directory.
+`prefix` to create a unique temporary directory. Due to platform
+inconsistencies, avoid trailing `X` characters in `prefix`. Some platforms,
+notably the BSDs, can return more than six random characters, and replace
+trailing `X` characters in `prefix` with random characters.
 
 The created folder path is passed as a string to the callback's second
 parameter.
@@ -4280,7 +4283,10 @@ added: v10.0.0
 
 Creates a unique temporary directory and resolves the `Promise` with the created
 folder path. A unique directory name is generated by appending six random
-characters to the end of the provided `prefix`.
+characters to the end of the provided `prefix`. Due to platform
+inconsistencies, avoid trailing `X` characters in `prefix`. Some platforms,
+notably the BSDs, can return more than six random characters, and replace
+trailing `X` characters in `prefix` with random characters.
 
 The optional `options` argument can be a string specifying an encoding, or an
 object with an `encoding` property specifying the character encoding to use.