[GRID] Optimize cell and region RECT calculations, if there

... is no per-column/row width/height.
[GRID] Fix grid_region_rect() and grid_cell_rect().

The previous implementation did not take scrolling into account.
[TABLE] Add documentation for column/row headers.
[doc] Fix many documenation errors.

 * Use @ref instead @c when possible.
 * Fix many typos and copy&paster errors.
 * Fix bad formatting of [in] and/or [out] @param specficators.