doc: west: add note about west update --keep-descendants option

The previous and short description wasn't enough for me to understand
the interesting trade-off of --keep-descendants, thanks to
@mbolivar-nordic for clarifying this on Slack.

For reference this option was added in west commit 11b8588303 part of
https://github.com/zephyrproject-rtos/west/pull/165/



Signed-off-by: Marc Herbert <marc.herbert@intel.com>