[lldb] Document the behaviour of IsValid for SBError (#170862)

This reverts commit d20d84fec5945fcc16aa6f63879e1458d4af9ea6.

Fixes #169788, but in a different way.

In which I changed an SBError use so that when the function succeeded,
IsValid on the SBError would be true.

This seemed to make sense but SBError acts differently to other SB
classes in this respect. For something like SBMemoryRegionInfo, if
IsValid() is false, you can't do anything with it.

However for SBError, IsValid() true only means there's some underlying
error object in there. If the SBError represents a success, there's no
need to put anything in there.

You can see this intent from a lot of its methods, many have handling
for IsValid() false.

This is not a bug but a misunderstanding of what IsValid means. Yes it
does function the way I expected for most classes, but it does not for
SBError and though that's not intuitive, it is consistent with how we
describe IsValid in the documentation.

So instead of changing that method's use of SBError I'm documenting this
initially counterintuitive behaviour in the SBError header and on the
website (https://lldb.llvm.org/resources/sbapi.html).