drivers: spi: nrfx: Update doc for RAM_BUFFER_SIZE Kconfig symbol

Update documentation for SPI_NRFX_RAM_BUFFER_SIZE Kconfig symbol
to reflect new usage of it. Now the symbol specifies size of RX buffer.
The change introducing support for RX buffer placed by a linker in
memory region defined in SPIM devicetree node is in a parent commit
of that one.

Signed-off-by: Adam Wojasinski <adam.wojasinski@nordicsemi.no>