Merge branch 'bugfix/spi_inconsistencies' into 'master'

SPI: Some fixes to docs and flag naming

Kolban noticed the flags for a transaction are not named like the documentation states. The flags as mentioned in the docs are more consistent, so this MR renames them to that. Also some additions to the docs wrt the SPI signals

Ref http://www.esp32.com/viewtopic.php?f=13&t=919&p=3976#p3976 and http://www.esp32.com/viewtopic.php?f=13&t=921&p=3975#p3975

See merge request !398