There won't be one, not in the sense you are thinking of.

What ships w/the bike is a generic manual aimed mainly at limiting liability.

The various parts on the bike are from different brands, and you can often go to the brand sites to find documentation. However, the more lower end and generic the parts, the harder it can be to find technical documentation for them.

Amazon will have books on bicycle mechanics. Common types of components such as mechanical disk brakes all work much the same. Mostly. But not always.

Shop mechanics will have access to documentation from their bicycle brands, and also to industry manuals such as Barnett's: https://www.bbinstitute.com/the-barnett-manual

Your Specialized dealer would be the best person to ask about the valve cap. It's a Specialized-branded item, and one would hope the dealer would have access to some sort of documentation on it.