Update "Introduction to Raster classes" document

[Nikos Alexandris]

In the Introduction to Raster classes, please the remove the incorrect short sentence

Both classes write sequentially.

from

The read access is row wise for RasterRow and RasterRowIO and additionally cached in the RowIO class. Both classes write sequentially.

Only RasterRow (that is only one class) that writes sequentially.

[veroandreo]

diff files with correct text are always welcome :)

[Nikos Alexandris]

Update pygrass_raster.rst

[Nikos Alexandris]

Diff added. Also ​https://github.com/GRASS-GIS/grass-ci/pull/8.

However, do we need to submit diffs for such minor cases? I would prefer to update such cases directly. It's good that we move to git because I don't need direct access to the core.

[veroandreo]

In 74313:

pygrass raster docs: correction, only RasterRow writes sequentially (provided by Nikos Alexandris), fixes #3766