getVastXml function is not mentioned in the docs

[robin-crazygames]

The getVastXml function, which is used in the localCacheGam.html example, is not mentioned anywhere in the docs.

It should probably be added to:

• The API reference: https://docs.prebid.org/dev-docs/publisher-api-reference.html
• https://docs.prebid.org/dev-docs/publisher-api-reference/setConfig.html#client-side-caching-of-vast-xml