Improve documentation for encryptedSignalSources

[dgirardi]

the JS userId module has some functionality to support "encrypted signals for publishers":

https://github.com/prebid/prebid.github.io/blame/eb6c9134294dcb062bdb10ce239294d2c324e145/dev-docs/modules/userId.md#L324-L341

From the documentation it's not clear what the encrypt and customFunc parameters do. It also makes no mention of a public API method getEncryptedEidsForSource:

https://github.com/prebid/Prebid.js/blob/127913bcb865629a437784cd8dd19bef82da3699/modules/userId/index.js#L771-L775

[muuki88]

@dgirardi should we take this?