Add tasks description

[cypok]

$ gradle tasks --all
...
Verification tasks
------------------
check - Runs all checks.
test - Runs the unit tests.

Other tasks
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compileJava - Compiles main Java source.
compileKotlin - Compiles the source set 'main'.kotlin.
compileTestJava - Compiles test Java source.
compileTestKotlin - Compiles the source set 'test'.kotlin.
jetBuild
jetProfile
jetRun
jetTestRun
processResources - Processes main resources.
processTestResources - Processes test resources.
startScripts - Creates OS specific scripts to run the project as a JVM application.
...

It would be great to add description to all jet*** tasks.