Document what public API means

Author: tkf

doc/src/base/base.md

@@ -15,6 +15,11 @@ Some general notes:
   * By convention, function names ending with an exclamation point (`!`) modify their arguments.
     Some functions have both modifying (e.g., `sort!`) and non-modifying (`sort`) versions.
 
+The behaviors of `Base` and standard libraries are stable as defined in
+[SemVer](https://semver.org/) only if they are documented; i.e., included in the
+[Julia documentation](https://docs.julialang.org/) and not explicitly marked as non-stable.
+See [API FAQ](@ref man-api) for more information.
+
 ## Getting Around
 
 ```@docs

doc/src/manual/faq.md

@@ -18,6 +18,28 @@ For similar reasons, automated translation to Julia would also typically generat
 
 On the other hand, language *interoperability* is extremely useful: we want to exploit existing high-quality code in other languages from Julia (and vice versa)!  The best way to enable this is not a transpiler, but rather via easy inter-language calling facilities.  We have worked hard on this, from the built-in `ccall` intrinsic (to call C and Fortran libraries) to [JuliaInterop](https://github.com/JuliaInterop) packages that connect Julia to Python, Matlab, C++, and more.
 
+## [Public API](@id man-api)
+
+### How does Julia declare public API?
+
+The only interfaces that are stable with respect to [SemVer](https://semver.org/) of `julia`
+version are the Julia `Base` and standard libraries interfaces described in
+[the documentation](https://docs.julialang.org/) and not explicitly marked as non-stable.
+Functions, types, and constants are not part of the public API if they are not included in
+the documentation, _even if they have docstrings_.
+
+### There is a useful undocumented function/type/constant. Can I use it?
+
+Please open an [issue](https://github.com/JuliaLang/julia/issues) or
+[pull request](https://github.com/JuliaLang/julia/pulls) to start a discussion for turning it
+into a public API before relying on it.
+
+### The documentation is not accurate enough. Can I rely on the existing behavior?
+
+Please open an [issue](https://github.com/JuliaLang/julia/issues) or
+[pull request](https://github.com/JuliaLang/julia/pulls) to start a discussion for turning the
+existing behavior into a public API.
+
 ## Sessions and the REPL
 
 ### How do I delete an object in memory?