remove mention that non-underscored symbols denote public API

[KristofferC]

Ref #40131 (comment)

This is barely used in Base nor in almost any package I've read at read so it seems odd to have in the style guide.

[tpapp]

I agree that it is misleading (as it could be taken to imply that names without _ are part of the API), nevertheless _ and doulble __ are used for something.

For me it usually signals the intention that something is really a very internal implementation detail like Base.Cartesian._nloops or Base.IteratorsMD.__inc. Since some people use it that way, maybe some reference to that could be kept in the style guide.

[vtjnash]

Having knowledge of some functions in base (_hypot) that "violate" this, I think we should just delete it. We occasionally use this to indicate a version of the function that is more "low-level" or "internal," but even that classification seems a judgement call, and not necessarily a stricture to highlight in the docs