t/chainlint/*.test: generalize self-test commentary

t/chainlint/*.test: generalize self-test commentary

The purpose of chainlint.sed is to detect &&-chain breakage only within
subshells (one level deep); it doesn't bother checking for top-level
&&-chain breakage since the &&-chain checker built into t/test-lib.sh
should detect broken &&-chains outside of subshells by making them
magically exit with code 117. However, this division of labor may not
always be the case if a more capable chainlint implementation is ever
developed. Beyond that, due to being sed-based and due to its use of
heuristics, chainlint.sed has several limitations (such as being unable
to detect &&-chain breakage in subshells more than one level deep since
it only manually emulates recursion into a subshell).

Some of the comments in the chainlint self-tests unnecessarily reflect
the limitations of chainlint.sed even though those limitations are not
what is being tested. Therefore, simplify and generalize the comments to
explain only what is being tested, thus ensuring that they won't become
outdated if a more capable chainlint is ever developed.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>