Document the default value for `mutateDocument`

[kberg]

I cannot run the tests, when I run npm install && npm run test I get

node:internal/crypto/hash:79
  this[kHandle] = new _Hash(algorithm, xofLen, algorithmId, getHashCache());
                  ^

Error: error:0308010C:digital envelope routines::unsupported
    at new Hash (node:internal/crypto/hash:79:19)
    at Object.createHash (node:crypto:139:10)
    at module.exports (/home/kberg/git/JSON-Patch/node_modules/webpack/lib/util/createHash.js:135:53)
    at NormalModule._initBuildHash (/home/kberg/git/JSON-Patch/node_modules/webpack/lib/NormalModule.js:412:16)
    at handleParseError (/home/kberg/git/JSON-Patch/node_modules/webpack/lib/NormalModule.js:463:10)
    at /home/kberg/git/JSON-Patch/node_modules/webpack/lib/NormalModule.js:495:5
    at /home/kberg/git/JSON-Patch/node_modules/webpack/lib/NormalModule.js:354:12
    at /home/kberg/git/JSON-Patch/node_modules/loader-runner/lib/LoaderRunner.js:373:3
    at iterateNormalLoaders (/home/kberg/git/JSON-Patch/node_modules/loader-runner/lib/LoaderRunner.js:214:10)
    at Array.<anonymous> (/home/kberg/git/JSON-Patch/node_modules/loader-runner/lib/LoaderRunner.js:205:4)
    at Storage.finished (/home/kberg/git/JSON-Patch/node_modules/enhanced-resolve/lib/CachedInputFileSystem.js:55:16)
    at /home/kberg/git/JSON-Patch/node_modules/enhanced-resolve/lib/CachedInputFileSystem.js:91:9
    at /home/kberg/git/JSON-Patch/node_modules/graceful-fs/graceful-fs.js:90:16
    at FSReqCallback.readFileAfterClose [as oncomplete] (node:internal/fs/read/context:68:3) {
  opensslErrorStack: [
    'error:03000086:digital envelope routines::initialization error',
    'error:0308010C:digital envelope routines::unsupported'
  ],
  library: 'digital envelope routines',
  reason: 'unsupported',
  code: 'ERR_OSSL_EVP_UNSUPPORTED'
}