build: clarify --upload once again for Linux boards

BUILD.md

@@ -75,22 +75,25 @@ list some basic and more used commands as example.
     ./waf --targets bin/arducopter --upload
     ```
 
-    On Linux you need to first configure the IP of the board you are going to upload
-    to. This is done on configure phase with:
+    For Linux boards you need first to configure the IP of the board you
+    are going to upload to. This is done on configure phase with:
 
     ```sh
     ./waf configure --board <board> --rsync-dest <destination>
     ```
 
-    What this does is to install to a temporary location and calling
-    `rsync <temp_install_location>/ <destination>`. Example (board and destination
+    The commands below give a concrete example (board and destination
     IP will change according to the board used):
 
     ```sh
     ./waf configure --board navio2 --rsync-dest root@192.168.1.2:/
     ./waf --target bin/arducopter --upload
     ```
 
+    This allows to set a destination to which the `--upload` option will upload
+    the binary.  Under the hood  it installs to a temporary location and calls
+    `rsync <temp_install_location>/ <destination>`.
+
     On Linux boards there's also an install command, which will install to a certain
     directory, just like the temporary install above does. This can be
     used by distributors to create .deb, .rpm or other package types: