eina: Cleanup documentation for Sparse Matrix.

eina: Cleanup documentation for Sparse Matrix.

Summary:
Revises some descriptions, polishes wording, makes punctuation and
formatting more consistent, and straightens out some convoluted grammar
here and there.

There are a few places where the parameter documentation was incorrectly
cut-and-pasted from other routines, so this is fixed.  Also, in
eina_matrixsparse_iterator_new() the docs say the elements are not
returned sequentially, but from the context and code I think it meant to
say they're not _consecutive_.

Subscribers: cedric

Differential Revision: https://phab.enlightenment.org/D5924

Reviewed-by: Cedric BAIL <cedric@osg.samsung.com>