docs(assert): document how to assert a captured exit code

[Chemaclass]

🤔 Background

Related #1169

The exit-code assertions read $?. Under set -e — which --strict turns on
for every test — that idiom cannot be used directly: the non-zero exit aborts
the test before the assertion runs, so the code has to be captured, and
capturing it sets $? to zero.

local code=0
some_command || code=$?

assert_general_error            # reads $?, the assignment's 0
assert_general_error "$code"    # $1 is never read by these functions

Both silently assert the wrong thing, and the second is the natural spelling.
The working form — assert_general_error "" "" "$code" — was undocumented.

💡 Changes

• Document the third-argument form in all four sections, noting that assert_exit_code takes its expected code in $1
• Appended at the end of each section: the bashunit doc renderer truncates a section, and inserting earlier silently dropped the existing assert_exec tip from CLI output. With the block at the end the doc snapshot is byte-identical

Not changing

Making $1 the captured code would fix the natural spelling, but
assert_general_error "$(some_cmd)" is an established idiom in this repo that
relies on $? and would silently change meaning for numeric output. That is a
contract change for a maintainer.