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: https://github.com/nodejs/node/pull/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>