[3.x] Document that Input.is_action should not be used during input-handling

[lawnjelly]

In most cases InputEvent.is_action* is more appropriate during input-handling.

Backport of #80185 .
Helps address #97526 .
Helps address #80158 .

Notes

• This confusion has come up in master and 3.x.
• Whether to go forward with this depends on whether we intend to use an alternative automated approach (e.g. Add scope parameter to Input.is_action_just_pressed() godot-proposals#10843)
• Ideally this documentation would be used in combination with a WARN_PRINT_ONCE or similar if e.g. Input.is_action_just_pressed() is used withing _input(), but that should be a separate PR.

[lawnjelly]

Will merge this for now, if we do change to an automated approach, this will be easy to change.