docs: clarify section on coroutine memory consumption (#3225)
docs: clarify section on coroutine memory consumption (#3225)

Updates the section "Coroutines are light-weight" to clarify its
comparison of the resource-intensiveness of coroutines and threads. The
content is much the same.

This commit also makes the code sample in this section runnable and
omits the surrounding runBlocking block.
Kotlin/kotlinx.coroutines,Kotlin/kotlinx.coroutines,Kotlin/kotlinx.coroutines,Kotlin/kotlinx.coroutines,Kotlin/kotlinx.coroutines