Clarify items for bars should be sorted

[PaulWessel]

See #7021 for background. This PR adds some language to indicate the multiband values should be sorted if +v is used. Closes #7021.

[PaulWessel]

@joa-quim, what can we do to avoid this "internal compiler error" shits in the CI? Id says it happens at line 6338 in gmt_support.c which is inside an old function gmtlib_detrend which I dont thin have changed, and of course you do not get these errors. Can you see anything funny in the actual command switches the CI uses in that log? It is not good that every commit triggers this failed test.

[joa-quim]

I have a slightly older version of the compiler and this is telling me to not let it update. We can try reversing the order of the if else test. We could also try report this error but honestly after experience and stupid answers when I reported that sort bug that they refused to accept as a bug (C standard undefined behavior, f) I have no more patience for them.

[yvonnefroehlich]

From my side, this addition to the documentation is OK.