Remove mention of bulk threadpool in examples (#41935)

The `bulk` threadpool is now called `write`, but `bulk` is still
used in some examples. This commit fixes that.

Also, the only way `threadpool.bulk.write: 30` is a valid increase in the size
of this threadpool is if you have 29 processors, which is an odd number of
processors to have. This commit removes the "more threads" bit.

docs/reference/modules/threadpool.asciidoc

@@ -52,13 +52,14 @@ There are several thread pools, but the important ones include:
     Mainly for java client executing of action when listener threaded is set to true.
     Thread pool type is `scaling` with a default max of `min(10, (# of available processors)/2)`.
 
-Changing a specific thread pool can be done by setting its type-specific parameters; for example, changing the `bulk`
-thread pool to have more threads:
+Changing a specific thread pool can be done by setting its type-specific
+parameters; for example, changing the number of threads in the `write` thread
+pool:
 
 [source,yaml]
 --------------------------------------------------
 thread_pool:
-    bulk:
+    write:
         size: 30
 --------------------------------------------------
 
@@ -87,7 +88,7 @@ full, it will abort the request.
 [source,yaml]
 --------------------------------------------------
 thread_pool:
-    bulk:
+    write:
         size: 30
         queue_size: 1000
 --------------------------------------------------