update for attribute documentation

[edhartnett]

Fixes #1512

Previously we had been trying to get by with one documentation block for all the different types of nc_get_att_, and another for nc_put_att_. But this was an imperfect solution.

Some problems:

• many functions do not get correct links throughout the documentation (though some of them are working).
• documentation had to be confusingly general.
• nearly-duplicate documentation blocks were needed to handle typeless and string type functions.

So, after this PR, each att fucntion gets its own doxygen documentation, which makes doxygen happy. I have edited the documentation to take repeated parts out and put them in the member group for "Reading Attributes" and "Writing Attributes" and have added achors to them so they can be referred to from each function definion, which now include (for the gets):

 * Also see @ref getting_attributes "Getting Attributes"
 *

Now we have a place to put documentation about getting attributes that applies to all types, and also the function documentation for each type, to note type-specific info as with text or string types.

[edhartnett]

@WardF if we could get this merged soon, similar fixes are needed for the var functions, and I would like to get them in before your next release.,;-)

[edhartnett]

@WardF working late on a Friday night! ;-)