improves description about the rabin chunker

License: MIT
Signed-off-by: Enrique Erne <enrique.erne@gmail.com>

core/commands/add.go

@@ -75,10 +75,11 @@ You can now refer to the added file in a gateway, like so:
 The chunker option, '-s', defines the chunking strategy for how
 ipfs puts larger files into blocks. Blocks with same content can
 be deduplicated. The default is a fixed block size of
-256 * 1024 bytes, 'size-262144'. A variable block size can be
-specified by using the rabin chunker, rabin-[min]-[avg]-[max].
-Using different chunking strategies will produce different hashes 
-for the same file.
+256 * 1024 bytes, 'size-262144'. Alternatively, you can use the
+rabin chunker for content defined chunking by specifying
+rabin-[min]-[avg]-[max] (where min/avg/max refer to the resulting 
+chunk sizes). Using other chunking strategies will produce
+different hashes for the same file.
 
   > ipfs add --chunker=size-1024 ipfs-logo.svg
   added QmZ9KNxxeeLCPrdceBjvVcmSyLbUWMF4ceKDP8yboLNoHT ipfs-logo.svg