Add description about interactive thread pool (#1054)

* Add description about interactive thread pool

* Update docs/src/server.md

---------

Co-authored-by: Jacob Quinn <[email protected]>

Author: AtsushiSakai

docs/src/server.md

@@ -107,3 +107,9 @@ Lower-level core server functionality that only operates on `HTTP.Stream`. Provi
 ## Log formatting
 
 Nginx-style log formatting is supported via the [`HTTP.@logfmt_str`](@ref) macro and can be passed via the `access_log` keyword argument for [`HTTP.listen`](@ref) or [`HTTP.serve`](@ref).
+
+## Serving on the interactive thead pool
+
+Beginning in Julia 1.9, the main server loop is spawned on the [interactive threadpool](https://docs.julialang.org/en/v1.9/manual/multi-threading/#man-threadpools) by default. If users do a Threads.@spawn from a handler, those threaded tasks should run elsewhere and not in the interactive threadpool, keeping the web server responsive.
+
+Note that just having a reserved interactive thread doesn’t guarantee CPU cycles, so users need to properly configure their running Julia session appropriately (i.e. ensuring non-interactive threads available to run tasks, etc).