program: Update instruction comments

[febo]

Problem

There are a couple of issues in the comments:

• Incorrect account indices in Trim and Close.
• Extend and Trim do not specify that metadata accounts need to be mutable.

Solution

Update the comments fixing account indices and explicitly mentioning that immutable metadata accounts cannot be extended nor trimmed.

[lorisleiva]

👌

[febo]

@lorisleiva @joncinque Sorry, just realised that Extend needed an update as well.