docs(skipping): warn that '&& return' breaks a conditional skip

[Chemaclass]

🤔 Background

Related #1213

The two skip families need opposite call shapes, and nothing said so:

function	stops itself	needs && return	with && return when not skipping
skip	no — marks only	yes	n/a
skip_if / skip_unless*	yes (exit 0)	no	test ends having asserted nothing

bashunit::skip_if "[ 1 -eq 2 ]" "not met" && return   # condition false…
assert_same 1 1                                       # …yet this never runs

skip_if ends on a false if, so it returns 0, && return fires, and the
test finishes empty. It surfaces as risky rather than passing — the only
hint you get.

💡 Changes

• Warning in docs/skipping-incomplete.md next to the conditional skips

Found by writing the idiom myself while executing the guide: three tests came
out risky instead of passing. The documented examples were already correct —
what was missing is why the shape differs.