Undocumented public API in PBJS userId module (`registerSignalSources`)

[dgirardi]

prebid/Prebid.js#8117 introduced pbjs.registerSignalSources(). Its use seems to be necessary for some features to work (see prebid/Prebid.js#8532), but there is no mention of it in the documentation.

For reference, the original docs PR: #3643

@nitin0610 FYI