docs: add @__FUNCTION__ to docs and news

Author: MilesCranmer

NEWS.md

@@ -6,6 +6,7 @@ New language features
 
   - New `Base.@acquire` macro for a non-closure version of `Base.acquire(f, s::Base.Semaphore)`, like `@lock`. ([#56845])
   - New `nth` function to access the `n`-th element of a generic iterable. ([#56580])
+  - New `@__FUNCTION__` macro that returns a reference to the innermost enclosing function. ([#58909])
   - The character U+1F8B2 🢲 (RIGHTWARDS ARROW WITH LOWER HOOK), newly added by Unicode 16,
     is now a valid operator with arrow precedence, accessible as `\hookunderrightarrow` at the REPL.
     ([JuliaLang/JuliaSyntax.jl#525], [#57143])

doc/src/base/base.md

@@ -483,6 +483,7 @@ Base.@__MODULE__
 Base.@__FILE__
 Base.@__DIR__
 Base.@__LINE__
+Base.@__FUNCTION__
 Base.fullname
 Base.names
 Base.isexported