Fix documentation: thread pool of main thread (JuliaLang#53388)

See
JuliaLang#53217 (comment)

(cherry picked from commit 8425b0e)

doc/src/manual/multi-threading.md

@@ -116,8 +116,8 @@ julia> using Base.Threads
 julia> nthreadpools()
 2
 
-julia> threadpool()
-:default
+julia> threadpool() # the main thread is in the interactive thread pool
+:interactive
 
 julia> nthreads(:default)
 3
@@ -133,6 +133,10 @@ julia> nthreads()
     The zero-argument version of `nthreads` returns the number of threads
     in the default pool.
 
+!!! note
+    Depending on whether Julia has been started with interactive threads,
+    the main thread is either in the default or interactive thread pool.
+
 Either or both numbers can be replaced with the word `auto`, which causes
 Julia to choose a reasonable default.