Update rsync.md (#718)

Clarify folder/folder-content syncing.
Add options --partial and -P, since partial helps with network errors and -P is convenient.

rsync.md

@@ -9,7 +9,10 @@ weight: -1
 {: .-prime}
 
 ```bash
+# syncing folder src into dest:
 rsync -avz ./src /dest
+# syncing the content of src into dest:
+rsync -avz ./src/ /dest
 ```
 
 ### OSX
@@ -25,6 +28,7 @@ rsync -avz ./src /dest
 ```bash
 -z, --compress
 -n, --dry-run
+    --partial   # allows resuming of aborted syncs
 ```
 
 ### Display options
@@ -34,6 +38,7 @@ rsync -avz ./src /dest
 -v, --verbose
 -h, --human-readable
     --progress
+-P                     # same as --partial --progress
 ```
 
 ### Skipping options