doc: return type of crypto.getFips() may change

Document that the return type of `crypto.getFips()` may change in a
future semver-major release from a `number` to a `boolean`.

PR-URL: #32580
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Signed-off-by: Richard Lau <riclau@uk.ibm.com>

doc/api/crypto.md

@@ -2309,7 +2309,8 @@ added: v10.0.0
 -->
 
 * Returns: {number} `1` if and only if a FIPS compliant crypto provider is
-  currently in use, `0` otherwise.
+  currently in use, `0` otherwise. A future semver-major release may change
+  the return type of this API to a {boolean}.
 
 ### `crypto.getHashes()`
 <!-- YAML