RFR: 8146948: Add help information to log tags

[Kim Barrett]

> On May 10, 2016, at 2:57 AM, Marcus Larsson <marcus.larsson at oracle.com> wrote:
> 
> Updated webrev:
> http://cr.openjdk.java.net/~mlarsson/8146948/webrev.01/
> 
> Incremental:
> http://cr.openjdk.java.net/~mlarsson/8146948/webrev.00-01/

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src/share/vm/logging/logTagSetDescriptions.hpp

I'm not sure why the macros are here rather than in the .cpp file.  I
don't think there's a technical reason to put the descriptions in the
header.  There might be a consistency of style reason, as other
similar macrology in the logging code puts the lists in the headers,
but I think in all of the other cases there is a technical necessity
for that.

If the macros (including the actual descriptions) were moved to the
.cpp file, only a forward declaration of LogTagSet would be needed in
the header; the include of logTagSet.hpp could be moved to the .cpp
too.

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I'm OK with this change as is, or with the description macros moved as
suggested.  I've a preference for the latter, assuming there isn't
some issue with doing so that I've overlooked.  Your call.  I don't
need a new webrev if you move the macros.