Improved documentation around sequence of saveState and getState

Upon writing my first action, I found it surprising that after issuing a call like core.saveState('foo', 'bar') I could not immediately core.getState('foo') back from the environment. After reading the code a bit, I sort of understand why. Although core.getState is reading directly from the environment variables, core.saveState is writing to stdout.

The docs for sending values to pre and post actions implies this. (emphasis mine)

The STATE_processID variable is then exclusively available to the cleanup script running under the main action.

It would be nice if the documentation around core.saveState and core.getState explicitly mentioned this. I didn't initially expect this behavior.

Start with the documentation for core.saveState and core.getState and compare it with the linked GitHub Actions workflow-commands reference. Update the documentation to explain that saveState writes to stdout and its value is not immediately available through getState, including the pre/post action scope. Done means the sequence and limitation are explicit.