Update doxygen for istream::tellg() to reflect PR/26211?

[Paolo Carlini]

... I reconsidered this issue and realized that what is really missing 
from the Doxygen comment for these functions is a note about eofbit 
being cleared first or not. Indeed N3168 does not correct for tellg the 
surprising behavior noticed in US 137 / US 139 (when DR60 is fully 
implemented and a sentry constructed also for the seekg / tellg 
functions, as finally happens in GCC 4.6) by the provision of clearing 
eofbit first. I'm going to apply the below then.

Paolo.

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