Add text to debug doc about multiple invocations

[philrz]

What's Changing

Disclosing out-of-order output expectations in the debug operator docs.

Why

When doing final verification of the debug operator, I revisited the recreation of the jq example shown in #5181 and noticed something for the first time in our implementation: Whereas jq's debug outputs always appear in the relative order that matches their appearance in the pipeline:

$ jq -n '1 as $x | 2 | debug("Entering function foo with $x == \($x)", .) | (.+1)'
["DEBUG:","Entering function foo with $x == 1"]
["DEBUG:",2]
3

In our equivalent, the order of the debug outputs can vary. I understand this to be an expected side effect of our implementation, but I also expect this might look like a bug to some users, so I figure a disclosure might help.

$ zq 'const x=1 yield 2 | debug f"DEBUG: Entering function foo with x == {x}" | debug f"DEBUG: {this}" | yield this+1'
3
"DEBUG: 2"
"DEBUG: Entering function foo with x == 1"

$ zq 'const x=1 yield 2 | debug f"DEBUG: Entering function foo with x == {x}" | debug f"DEBUG: {this}" | yield this+1'
3
"DEBUG: Entering function foo with x == 1"
"DEBUG: 2"