KT-51515 cleanup (#3649)

* Fix comment about 100k coroutines in cancellation and timeouts section

Changed to 10K consistently with the code.

* Make "Coroutines are light-weight" example runnable

Reduce the number of coroutines to 50K - works on playground.
Also, change the working about what happens with threads.

Co-authored-by: Danil Pavlov <danil.pavlov@jetbrains.com>

docs/topics/cancellation-and-timeouts.md

@@ -424,7 +424,7 @@ fun main() {
 If you run the above code, you'll see that it does not always print zero, though it may depend on the timings 
 of your machine. You may need to tweak the timeout in this example to actually see non-zero values. 
 
-> Note that incrementing and decrementing `acquired` counter here from 100K coroutines is completely thread-safe,
+> Note that incrementing and decrementing `acquired` counter here from 10K coroutines is completely thread-safe,
 > since it always happens from the same thread, the one used by `runBlocking`.
 > More on that will be explained in the chapter on coroutine context.
 > 

docs/topics/coroutines-basics.md

@@ -250,33 +250,34 @@ Done
 Coroutines are less resource-intensive than JVM threads. Code that exhausts the
 JVM's available memory when using threads can be expressed using coroutines
 without hitting resource limits. For example, the following code launches
-100000 distinct coroutines that each wait 5 seconds and then print a period
+50,000 distinct coroutines that each waits 5 seconds and then prints a period
 ('.') while consuming very little memory:
 
 ```kotlin
 import kotlinx.coroutines.*
 
 fun main() = runBlocking {
-    repeat(100_000) { // launch a lot of coroutines
+    repeat(50_000) { // launch a lot of coroutines
         launch {
             delay(5000L)
             print(".")
         }
     }
 }
 ```
-<!-- While coroutines do have a smaller memory footprint than threads, this
-example will exhaust the playground's heap memory; don't make it runnable. -->
+{kotlin-runnable="true" kotlin-min-compiler-version="1.3"}
 
 > You can get the full code [here](../../kotlinx-coroutines-core/jvm/test/guide/example-basic-06.kt).
 >
 {type="note"}
 
-<!--- TEST lines.size == 1 && lines[0] == ".".repeat(100_000) -->
+<!--- TEST lines.size == 1 && lines[0] == ".".repeat(50_000) -->
 
 If you write the same program using threads (remove `runBlocking`, replace
 `launch` with `thread`, and replace `delay` with `Thread.sleep`), it will
-likely consume too much memory and throw an out-of-memory error.
+consume a lot of memory. Depending on your operating system, JDK version, 
+and its settings, it will either throw an out-of-memory error or start threads slowly 
+so that there are never too many concurrently running threads. 
 
 <!--- MODULE kotlinx-coroutines-core -->
 <!--- INDEX kotlinx.coroutines -->

kotlinx-coroutines-core/jvm/test/guide/example-basic-06.kt

@@ -8,7 +8,7 @@ package kotlinx.coroutines.guide.exampleBasic06
 import kotlinx.coroutines.*
 
 fun main() = runBlocking {
-    repeat(100_000) { // launch a lot of coroutines
+    repeat(50_000) { // launch a lot of coroutines
         launch {
             delay(5000L)
             print(".")

kotlinx-coroutines-core/jvm/test/guide/test/BasicsGuideTest.kt

@@ -55,7 +55,7 @@ class BasicsGuideTest {
     @Test
     fun testExampleBasic06() {
         test("ExampleBasic06") { kotlinx.coroutines.guide.exampleBasic06.main() }.also { lines ->
-            check(lines.size == 1 && lines[0] == ".".repeat(100_000))
+            check(lines.size == 1 && lines[0] == ".".repeat(50_000))
         }
     }
 }