rsync -avPX /mnt/user/documents/ /mnt/other_documents/
Do not forget ending slashes on directories to transfer the contents of source into the destination.
Leaving off trailing slashes causes the source folder itself to sync into the destination path.
a - equals rlptgoD below:
r - recursive recurse into directories
l - links copy symlinks as symlinks
p - perms preserve permissions
t - times preserve modification times
g - group preserve group
o - owner preserve owner (super-user only)
D - same as --devices --specials
--devices preserve device files (super-user only)
--specials preserve special files
v - verbose increase verbosity
P - same as --partial --progress
--progress show progress during transfer
--partial keep partially transferred files
X - xattrs preserve extended attributes
W - whole-files copy files whole (w/o delta-xfer algorithm)
S - sparse handle sparse files efficiently
(VM hdd's and such)