Clarify post_build_script and after_script execution behavior

What does this MR do?

Clarify post_build_script and after_script execution behavior

post_build_script is skipped when earlier scripts in its shell context fail. after_script runs in a separate context and is unaffected.

Why was this MR needed?

To clarify current behaviour in the docs.

What's the best way to test this MR?

Documentation update.

What are the relevant issue numbers?

User defined after_script is executed after fai... (#3116 - closed)