Hi,
I improved the r.mapcalc manual a bit in r57997. The change is not in online manual yet but you can check it locally or in trac.
The change is trying to clarify how it works with the MASK and r.mapcalc. This was discussed in the ticket #2032 and later only on grass-dev ([GRASS GIS] #2032: r.mapcalc does not always respect MASK).
Moreover, the issues with “b = a + 3” and backwards compatibility with GRASS 6 are discussed in Notes instead of the beginning.
Please review the changes. Feel free to add or change the manual more. Because r.mapcalc is part of the core functionality, I think that good documentation is important.
Vaclav
https://trac.osgeo.org/grass/changeset/57997
http://grass.osgeo.org/grass70/manuals/r.mapcalc.html