Docs: Clarify statement about the API

[DilumAluthge]

If I understand correctly, if I have an object of type TransformedLogDensity, then the official way of getting the transformation or log_density_function fields is by doing ℓ.transformation or ℓ.log_density_function, respectively.

This pull request edits the docstring for TransformedLogDensity to emphasize that this is part of the stable public API and will not change in a non-breaking release.

[tpapp]

I thought it was implied that whatever that is part of the API cannot change in a breaking way without it being a non-breaking change.

I am not sure I understand the distinction between "stable API" and "API", can you please clarify?

[codecov]

Codecov Report

Merging #66 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #66   +/-   ##
=======================================
  Coverage   96.96%   96.96%           
=======================================
  Files           5        5           
  Lines          66       66           
=======================================
  Hits           64       64           
  Misses          2        2

Impacted Files	Coverage Δ
src/LogDensityProblems.jl	96.87% <ø> (ø)	⬆️

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[DilumAluthge]

I've changed "stable public API" to "public API".

I usually think of a package's API as being divided into two parts:

1. The public API. This is stable and follows semantic versioning, i.e. breaking changes to the public API can only be made when you bump the major version number.
2. The private API, also known as the internal API. You are allowed to make breaking changes to the private API in minor version bumps.

Generally, I assume that names that are exported are part of the public API, while names that are not exported are part of the private/internal API.

Also, as a general rule of thumb, I assume that field names of a struct are part of the private/internal API.

But in this case, it looks like the .transformation and .log_density_function are part of the public API, not the private/internal API. So I wanted to make that distinction.

I guess to me, "API" means it could either be the public API or the private/internal API.

[tpapp]

Thanks for clarifying this. I agree with everything you wrote above, except that for me an unqualified "API" means "public API".