Mention getOnlyElement() as an alternative in the Javadoc for getFirst().
Mention getOnlyElement() as an alternative in the Javadoc for getFirst().

This came up in my Java readability review ([]as I had been
aware of getFirst() but not of getOnlyElement(). Indeed, I think many of the
cases where it's valid to only look at one element from an iterator are
actually cases where there's only one element in the iterator, and so I suspect
I won't be the only person who's using getFirst() when they are really looking
for getOnlyElement() instead.

This CL also reformats the rest of the Javadoc for getFirst() to 100 characters
to be consistent with the new paragraph that I added.
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MOE_MIGRATED_REVID=126073627

DavesMan/guava,tli2/guava,google/guava,rgoldberg/guava,mosoft521/guava,xasx/guava,google/guava,rgoldberg/guava,typetools/guava,Xaerxess/guava,berndhopp/guava,typetools/guava,mosoft521/guava,EdwardLee03/guava,Xaerxess/guava,xasx/guava,tli2/guava,berndhopp/guava,EdwardLee03/guava,mosoft521/guava,DavesMan/guava,google/guava