clean up the documentation of threads (#21067)

* clean up the documentation of threads

* cleanup

doc/manual.md

@@ -8424,23 +8424,12 @@ This is only useful if the program is compiled as a dynamic library via the
 `--app:lib`:option: command-line option.
 
 
-
 Threads
 =======
 
-To enable thread support the `--threads:on`:option: command-line switch needs to
+The `--threads:on`:option: command-line switch is enabled by default. The [typedthreads module](typedthreads.html) module then contains several threading primitives. See [spawn](manual_experimental.html#parallel-amp-spawn) for
-be used. The [system module](system.html) module then contains several threading primitives.
-See the [channels](channels_builtin.html) modules
-for the low-level thread API. There are also high-level parallelism constructs
-available. See [spawn](manual_experimental.html#parallel-amp-spawn) for
 further details.
 
-Nim's memory model for threads is quite different than that of other common
-programming languages (C, Pascal, Java): Each thread has its own (garbage
-collected) heap, and sharing of memory is restricted to global variables. This
-helps to prevent race conditions. GC efficiency is improved quite a lot,
-because the GC never has to stop other threads and see what they reference.
-
 The only way to create a thread is via `spawn` or
 `createThread`. The invoked proc must not use `var` parameters nor must
 any of its parameters contain a `ref` or `closure` type. This enforces

lib/std/typedthreads.nim

@@ -9,12 +9,6 @@
 
 ## Thread support for Nim.
 ##
-## Nim's memory model for threads is quite different from other common
-## programming languages (C, Pascal): Each thread has its own
-## (garbage collected) heap and sharing of memory is restricted. This helps
-## to prevent race conditions and improves efficiency. See `the manual for
-## details of this memory model <manual.html#threads>`_.
-##
 ## Examples
 ## ========
 ##